I'm new here so keep that in mind...
I have a disappearing edge on my pool that holds about 1600 gallons. Water flows over the top of it (over the coping pavers) and into the lower pit (that's what I call it at least). The pit has two drains in it that run back to a single pump, which pulls water out of the lower pit and sends it atop the waterfall, which cascades down, flows back into the pool, and then flows over the disappearing edge back into the lower pit (think of it as a closed loop system).
I noticed that, when the pump was turned off, the lower pit's water level would rise on it's own around 1" an hour (even if the water cascading over the disappearing edge and over the coping pavers), which may not sound like a big deal, but it actually is for a variety of reasons). At first I suspected the water was coming back into the lower pit from the 2 drains where the pump sucks water out, and was potentially back-flowing into the lower pit from the drains so I had the check-valve replaced (whatever that valve is called that stops water from going in the reverse direction). But that still didn't fix it, and anytime that pump was off, I'd tsill gain roughly 1" per hour in the lower pit.
It turned out that water was getting into the lower pit from mortar cracks that were roughly 1-2" underneath the coping pavers themselves between the main pool and the lower pit. In short, anytime the main pool's water level was at normal level (or within 1-2" of normal level) water was getting into the lower pit, not just from going OVER the pavers like it's designed to, but because the water was even going UNDER the pavers in cases where it wasn't high enough to flow over the top of the coping pavers.
To fix it, I needed to re-mortar all the coping areas that lead to the lower pit from the main pool (re-mortar both the pool side -and- lower pit side, not just one side!!) Once I did that, I no longer saw a 1" per hour increase in the lower pit's water level when the pump that sucks water OUT of the pit was turned off.
Not saying that's your issue but, ugh.., that one issue took me about 1-2 months to actually track down.
QUICK EDIT: I should mention, my pool is about 10-11 years old now, so cracks in mortar are becoming more common due to its age and I need to re-mortar even MORE areas than just the one area I fixed leading to the lower pit. but I think you should check this at least, because your symptom is very similar to what mine was , especially relating to when the pump is on/off!