It's the risk of metal staining especially against plaster surfaces and also the risk of turning blond hair greenish. If those risks weren't present, then we'd likely put copper ions in the same bucket as Polyquat 60 for algae control. It's not necessary because maintaining a proper Free Chlorine (FC) to Cyanuric Acid (CYA) ratio prevents algae growth.
With CYA in the water, the active chlorine level is low at around 0.06 ppm FC equivalent with no CYA for our minimums for non-SWG pools. That 7.5% FC/CYA ratio is what is needed to prevent green and black algae growth regardless of algae nutrient (phosphate, nitrate) level. If one wants to go below that FC/CYA level, then one would need to use an algaecide to prevent algae growth, though the disinfection rate would also be lowered as well. Though clearly in your case your copper ions aren't working to prevent algae at this point in time, possibly because their level has dropped or because you have more algae nutrients now and the copper level isn't sufficient (while earlier that level was enough when algae nutrient levels were lower).
I think you've got stuck in your brain that a high FC number means something bad in terms of having a lot of chlorine. It doesn't. The FC is the chlorine reserve since most of it is bound to CYA and inactive. The amount of chlorine that is unbound to CYA and is what disinfects, kills algae, and oxidizes swimsuits, skin, and hair is that very low 0.06 ppm amount. So if the only reason you are trying to go lower is this incorrect thought of the FC number meaning something that it really doesn't, then try to get your head around that. The only issue for a high FC would be if you were to drink lots and lots of pool water, but to get to the EPA limit of 4 ppm FC with 2 quarts a day for a lifetime, at 8 ppm FC you'd still have to drink a quart of pool water every day for the rest of your life and you'd still be at the 1 in a million increased cancer risk the EPA uses for their limits. And of course, you aren't drinking pool water in any such quantities so this is really irrelevant.
As for disinfection, as described in detail in
this post, copper alone does not kill fecal bacteria and copper and silver together kill bacteria slowly, do not effectively inactivate viruses, and do not kill protozoa or inactivate their oocysts. The reason the metal ions work for NASA is 1) there is not a constant introduction of fecal matter with new bacteria, viruses, protozoan oocysts, etc. in the drinking water supply and 2) there is plenty of time for a slow kill of any residual pathogens in the drinking water. In a swimming pool without proper disinfection there is not only the risk of uncontrolled bacterial growth but also the risk of person-to-person transmission of disease. That is why the EPA requires a fast-acting disinfectant in the bulk pool water. The use of off-line systems such as UV or ozone do nothing to prevent pathogen transmission or even growth if stuck on pool walls and not circulated. The use of slow-acting metal ions do not kill quickly enough to meet EPA's Swimming Pool Disinfectants standard
DIS/TSS-12. The only EPA-approved disinfectant chemical types for swimming pools are chlorine, bromine, and Baquacil/biguanide/PHMB.
Ignore the APSP, NSPI, CDC, EPA, state code, and everything else you've read (except for peer-reviewed scientific papers in respected journals) because none of them understand the FC/CYA relationship even though it's been known definitively since at least 1974. You can read more about this relationship in the "Chlorine/CYA Relationship" section of the thread
Certified Pool Operator (CPO) training -- What is not taught. Specifying an FC range without referencing the CYA is ridiculous since the FC/CYA ratio (when CYA is present) is what determines the level of active chlorine that disinfects and oxidizes.
As for why your pH rose so quickly, we simply do not know since you haven't told us anything that would give us a clue as to the reason (you gave us info, but none of it is consistent with a rise in pH). If you see any breaks in your plaster with volcano-like formation of calcium carbonate on the floor or streaks of it on the walls then I'd say you had
Calcium Nodules in your pool and the calcium hydroxide that gets exposed to the water and converted to calcium carbonate can raise the pH. If you had a change in aeration (rainfall, waterfalls, spillovers, fountains, pool usage including splashing) or longer pump run time then that can have more carbon dioxide outgassing causing the pH to rise more. If you used a different source of chlorinating liquid or bleach, then it might have more excess lye in it though that usually isn't enough to be noticeable in such a short time. If you used to use a net acidic chemical such as Dichlor, Trichlor, or non-chlorine shock (MPS) and this year stopped using it then perhaps your pH was kept in check with those chemicals and now it isn't. If you add a hypochlorite source of chlorine the pH will rise temporarily as the FC rises, but you compared pH on two days with the same FC level. If you accidentally added pH Up or similar product instead of the intended acid, then that would raise the pH quickly. Again, these are just guesses because nothing you mentioned would point to a reason for more rapidly rising pH that we normal know about.
You did notice, at least most recently, that the pool water was getting cloudy, possibly from algae, so maybe the algae growth is causing a rise in pH though normally we don't see that. However,
this paper implies that algae growth will tend to move the pH towards the 8.5-11.0 range so maybe that is what happened. If the cloudiness was from plaster deterioration (calcium nodules) then as I noted above that would raise the pH as well. However, you said that this entire season you've had to add more acid, not just recently. Perhaps you had nascent algae growth that wasn't yet visible and that tended to have the pH rise.
