there are two save types :
online saves that are only *visible* in game when game is connected to hellpoint online system
offline saves that are only *visible* in game when game is not connected to hellpoint online system
so for all those whos save seam to dissapear its probably you created/played online and when it appears to vanish its probably you are now disconnected from server or other way around (ware offline and now online)
solution ? carrefuly watch as game loads (not into map) there should be quite obvious symbol appear for second or few when game connects to online system , when it appears and you have multyplyer options available when playing - create dummy char with name "online" or something that you will recognise after checking that in load menu you still have char you just played . WHY ? so you can see more easily when you have online chars in load menu
then latter close gog galaxy when out of game, and launch it without gog galaxy so you will be sure you getting offline and create dummy char with name "offline" or something that you will recognise as not connected
result - you now always see in load menu whatever you are online or ofline due to those two dummy chars serving as improvised connection indicators

Just for those who "lost" their online saves permanently because they don't have an Internet connection, active GOG account, or Galaxy installed anymore – or moved the save from Windows to Linux like I did, because the Linux version of Hellpoint is always "offline" due to the lack of Galaxy for Linux. A big BOO to the devs for making Galaxy mandatory for online play.

However, you can "convert" an online save by changing the following entry in the respective ".hp" save file with a text editor that supports files with very long lines (over xx thousand chars long):

Change "user":"12345678901234567" (your number will be different, because that's your online ID) at nearly the very end of the file to "user":"Offline". Then you should be able to load the save in offline mode.

BUT be aware that it may be reverted to its former state from the dev's servers if you change it back to online by restoring your ID in the file. See the following forum link. So it seems like this is a one-way road without coming back to online without losing any progress that you made offline.

https://www.gog.com/forum/hellpoint/save_file_reverted_to_earlier_state_6_hours_of_progress_lost

edit: See here for the resp. save file locations on Windows and Linux. And backup any files before changing them!

https://www.pcgamingwiki.com/wiki/Hellpoint