Looking back at everything from the past week or two, something that caught my eye today is that your CYA is closer to 60, when before I believe the SLAM was based on 50. Doesn't seem like much, but it does require a slightly higher SLAM level. So in the previous attempts, if the FC should've been higher, it could explain why you were seeing some progress, only to have the tile revert back to color changes right afterwards. I know the FC was elevated for 24 hrs to rule-out Mustard Algae, but that is only for 24hrs after a successful SLAM. I understand the OCLT passed more than once, but in some unusual cases, if algae has become root-based into the pores of gunite/grout, it can be very difficult to penetrate. The tabs changing those stains seems to confirm.
With a CYA of "60-ish", if there is any chance it is in the higher end of the 60s, I would round up to 70 for future SLAM attempts. Also, we haven't talked about it much, but assuming you've inspected every inch of the pool for potential algae hiding places (ladder, behind light in niche, under skimmer lid, etc), when was the last time you did a thorough sand deep cleaning? If it's been a while, you might consider looking at the TFP
Deep Cleaning a Sand Filter. Perhaps something else is allowing that organic stuff to come back to life?? I'm looking at just about anything now.
Other than that, I have a feeling that organic stuff in the grout has its roots firmly in there and the ultimate remedy will be continued scrubbing and an elevated FC level, rounded-up to match the highest possible CYA in your water, and perhaps another MA level treatment based on that higher CYA.