Watermelon Check my edit pls :)
Depending on where one looks online, one might find estimates of lithium-ion lifetimes of 200 through 1000 cycles. In part it depends on how much reduction in capacity one is willing to accept before declaring the battery to be in need of replacement.
If the battery was showing sudden big drops in estimated state of charge at 200 cycles, that might not be out of the ordinary. Some batteries will do better than others, but that means some will do worse than others.
If the battery was showing sudden big drops in estimated state of charge at 100 cycles, that probably qualified as atypical at the time. I don't know what "wild fluctuations" means in terms of estimated state of charge, nor do I know what the numeric cycle count was during "early use" of the device.
Regardless of what the situation was prior to December 2024, a battery that currently has 500+ cycles might well show sudden drops in estimated state of charge due to age/wear. Or maybe something was odd about that battery or the charge controller "from birth".