I was just listening to some music, and I noticed a notification that said the MicroSD was in the process of ejecting. I waited and after a while, the player told me the MicroSD was corrupted. I noticed this happened after adjusting the volume using the physical buttons. It's possible it is a hardware issue, but it could also be coincidence.
I turned the player off, removed the MicroSD, and plugged it into my computer where it prompted to to a disk scan. After doing this, Windows reported that there were no errors. I put the MicroSD back in, turned the player on, and it detected the MicroSD properly.
EDIT: I've done some fiddling around, and it seems that I've been able to consistently reproduce this issue. I don't think it's a hardware issue. When not playing music for a while, the player goes into a sleep mode (power button light turns off). If the player is woken up and the volume is
lowered using the button while the screen is on, the MicroSD icon briefly appears in the notification tray.
If the player is woken up, the music is unpaused (order irrelevant), and the volume is
lowered using the button while the screen is on, the MicroSD icon appears for longer, resulting in either Android flagging the MicroSD as corrupt (remedied by restarting) or the MicroSD correctly being remounted. I've noticed that this happens when using Neutron or using Hiby Music, so it is probably not app dependent. I've noticed this happen with 2.5mm output, 3.5mm output, and no connected output.