There’s also an issue with Steam not installing Workshop file updates. I’ve played Unturned Classic on a community server recently and had no other choice of installing an update to this mod besides quitting the game client, unsubscribing and re-subscribing the mod I want to update (basically re-installing it).
This is typically more of a Steam/internet issue than an Unturned issue. Normally, Steam will sync with Steam Cloud after you close the game. If you close the game and then immediately relaunch the game, the files won’t have time to be synced.
You could wait longer between reopening the game after you close it, or disable Steam Cloud if you don’t want to use Steam Cloud.