Just to be clear, I am 100% aware that I have to use the OSD to get it to save.
When I first save (through the OSD), it works, but no matter what I do afterwards, it resets to that data when I hard-reset the core (not just the game). To alter the save file, I have to put the SD card in another device and change it.
My best guess is that the memory card isn't getting mounted on reboot
I am having the exact same issue. And it is not the cards not getting mounted. I've manually mounted a memory saved it, and looked at the SSD and it has not changed. Yeah currently the only way to save progress on the psx core is using save states, which sucks. I should also point out that I have a system that has worked for years, I have old saves that were working fine in a previous update but now can no longer be saved (updated).
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I went ahead and did some testing. I switched the core all the way back to PSX_20220511.rbf and I was still having the issue. Before I saw this message I thought the issue might be related to the game. So I tested another game I have and it worked fine.
Game with issue is:
Arc the Lad I
Game that worked fine:
Langrisser IV
I played through Arc The Lad I like a month ago. No problem with saving here. Have you tried deleting/renaming the memory card and let the core make a new one?
I played through the entire game as well since my last post, there are issues for me at least. Creating a new save file, using an old one, only using one game save, using older cores; none of that matters issue persists. What I did find out however is [arc the lad I] will not save if for example you bring up the game, do nothing, just save in game and tell core to save the memory card. Nothing will be saved, time stamp of memory card file not changed. If however you go into arc the lad and progress the story then save, it works fine. I've started arc the lad II, I copied [arc the lad I] save to be used by arc the lad II to convert. This worked totally fine, and in arc the lad II saving works fine using the original arc the lad I memory card file.