yeah, as Astra said, it is probably the controller becoming miscalibrated. your battery is not actually discharging this much, but the charge controller has become confused as to what voltage maps to what percentage
charging it to maximum, then letting it discharge until it shuts down a few times is a good way to refresh the cache of data such a charge controller will keep for battery level calculation (some devices esp. laptops have a feature to allow you to manually refresh the cache with a single discharge, but flipper apparently does not)
the reason that you would want to charge it to 40%-60% is that lithium-ion batteries lose capacity the slowest when left at around this amount of their capacity if not being charged or discharged. for general use, this is not the case (i’ve heard that using about 15% of the battery then recharging it is the best for preserving a battery that is in use, but idk how true this is)
but i guess you like using your flipper a lot (as anyone rational should) and so you don’t need to worry about the 40%-60% thing