Are both batteries from Canon, or the same manufacturer? Many third party batteries do not report “health” and other data to the camera
A battery grip works by alternating which battery is being used, so that the two batteries are drained at close to the same rate. The battery switching is done automatically by the firmware in the camera, not the grip.
it could right on the door step for switching. My DSLRs switch batteries based on discharge, not shot count. It will use a single battery until it has drained 3-5%, and then switch to the other battery.
If you remove the battery tray and then reinsert it, that may confuse the camera firmware’s ability to switch the load evenly. I would expect that it wouldn’t though.
Second battery began to drain after first battery dropped under 90%. Now both batteries are registering shutter count as I take photos.
Wow. It went all the way down to 90%? It could be switching after a 10% drain. That is a LOT more than my DSLRs, which switch around 5%.
I hope this means all will be well.