It's an incredibly frustrating bug; I noticed it ever since the Exodus 1.10 patch. Seemed like every new launch would wipe the Theater.json until I deleted the file, then seemed to improve to only daily that launching the game would wipe the contents of the file; it would keep loadouts for a day but wipe them on launch sometime within 24 hours.
Turned off Steam Cloud for Sandstorm and that seemed to keep them stored for an extra day, but then after, the named presets would still get wiped and only whatever was the last used loadout for a given class became its temp "default" in the Theater.json, if and only if that given class had been modified since the game was last launched (meaning, any class that hadn't been used in the last client session was still an unmodified default)
While in a new session, clicking on the actual "default" preset would reset any modified temp-default in the file back to the game's defined default—which don't ever appear in the Theater.json
Not sure if my efforts were really effectual or if any changes/workarounds actually had to do with server-side changes as I did notice playlists changing a couple of times over the past week or so of trying to troubleshoot.