As @FIREFOX says - this is something that happens to all Mavic batteries.
The reason is that LiPo batteries have a better life expectancy if they are not stored for long periods whilst fully charged, so they automatically discharge to ~60% after so many days of not being used.
In the Go4 app (not sure about the GoFly app for the Mini?) you can set the time before discharge starts … between 1 to 10 days.
I seem to recall this setting wasn’t available on the M2s, initially, but a recent firmware update has enabled it. (Perhaps an M2 owner would confirm?)
It’s worth adding that checking the battery level, without turning on the MP, resets the timer … at least, on the M1P batteries. (Same on M2/Mini, anyone?)
Not advisable! If it’s not flying things get rather hot inside! … as confirmed by the cooling fan (is there one in the Mini?) kicking in.
The rate of battery drain with the drone sat on the deck doing nothing, even if the motors are started and just idling, is very low compared to actually flying … so the time to discharge can be really a long time to have it all hot and bothered with the fan running.
Also check these posts for info.