When the maps exceed a certain size, you only have to change the file system on the device storage, internal or external (if it supports it). You will certainly not reduce the partition, you can only increase it. Then you would set it to exFAT.

I suppose that this was done after the firmware update. Maybe that partition is reserved for system files, maybe for boot, maybe for something else ... only Garmin can give the correct answer for sure. See the list of changes in the new firmware, if they have listed it at all.