Some games behave strangely, depending on whether you installed them through Galaxy or offline installer. It has to do with a player profile the game creates. Let me describe how Serious Sam 4 behaves.

1) At first, I installed it through offline installer. The game created a profile with all zeroes in its name. I could play the game without issues, save files worked.

2) Then an update came out. I didn't want to redownload everything again as the game is quite big, so I used Galaxy to update it. But this created a problem because once Galaxy touched the game, it created a new user profile (this time with numbers in its name) in the game files and the game swapped to using that profile with no way to point it back to the original profile.

3) This process was irreversible. From that point on, any time I started the game without Galaxy running, the saves would be gone (as it's Galaxy that is pointing the game to the user profile created by it) and the old profile from the offline installer would be ignored. The game would only ever look into the new profile created by the Galaxy update. The save files from between profiles are also not compatible, I tried copying the saves from the old profile into the new one, but it said the saves were corrupted, so they're probably profile bound.

4) I ended up just starting over, but it was definitely an annoying problem. Basically, if you ever install Serious Sam 4 through offline installers, NEVER let Galaxy touch it in any way as it will swap the game to a different profile and all your original progress will be lost.

Could be the same problem with this game. If you did the same thing, your only option would probably be trying to uninstall the game completely and installing again from the offline installer and then trying to use your old saves.

If you only ever installed and played through Galaxy, then I have no idea.