I'm on a private server with the Direwolf20 1.12.2 pack (1.9.0) installed and my AS progress resets every time I rejoin the server. We only have updated Portal Gun to 7.0.2, else the pack is the default one.
Please let me know if you need further information.

Multicraft, Bungeecord, Sponge, anything that would modify your player UUID on the server host?

Otherwise, need to take a look at the .astral files in the world save to see if the directory is write-protected.

Sorry to have bothered you, it was indeed a permissions issue. The server owner fixed it and everything is fine now.