I initially thought this was everyone but people on reddit say it doesn't happen for the steam version of the game and does for gog and gamepass. If you play a custom campaign from the dungeon maker, including the one added by the sorcerer update, and save your game on the gog or gamepass versions of the game, once you exit the game the save can't be loaded. It will have no picture in the list of saved games on the main menu, and trying to load it will cause the game to crash with a message saying the save is corrupted. If trying to reproduce the bug, while playing on the gog or gamepass client, you must create a custom game, save it, completely exit the game to desktop, then start the game again to reproduce the bug.
As a workaround, if you start a new custom game on the same level (might be any custom game I only tested with creating the exact same level), you can then load the save normally with no errors. You don't need to save your new custom game, just create it then you can load your other save directly in the game or exit back to the menu and load it from there. Loading of custom games will work normally until you exit the game completly, then you'll need to do this workaround again.
I have the gog version of the game so thats the only one I have tested, but based on posts on reddit I expect gamepass behaves identically, possibly any other non steam clients.