I will go along with rebooting being the issue. (I read the suggested long 536 thread.) Rebooting makes sense given that the volume has changed.
I will say I've noticed this problem before with other firmware versions, but thought maybe I didn't set the record feature. But turning down the volume was something new, and having the volume restored totally fits the reboot scenario.
I do not believe the FAT32 limitations come into play. There is a 64K (roughly) limit on the number of files per directory, but the recorder only puts 100 files in any directory. The format should handle 4Gbytes file size, though some reference 2Gbytes. But no audio recording will get that large. The 2Gbyte transition should not be an issue with the card.
I'm using a 32Gbyte card and have filled about 8% of it. The card is a Toshiba rated at 30mbps. (I don't know what came over me since I usually buy the no-name stuff.) Blaming the card is simply a rat hole to waste the users time. (Think of it as a Kellyanne Conway question deflection.) Software people blame hardware. Hardware people blame software. Customer gets stuck in the middle.
I will get the latest firmware, but personally I believe their is a problem in the firmware that has been around for a long time. When the dust settles, some instruction of the code will have written to the wrong place, some watchdog scheme forces the reboot.
Lots of good feedback in the thread, so thanks all.