In Singleplayer if you have supplies stored inside an Arsenal service, they either do not get saved by the game, or they don't get loaded by it during load time. The supplies are lost. You can easily try it:

1. Start a new singleplayer game.

2. Reach the rank of Corporal and build an Arsenal at any base.

3. Put supplies inside it.

4. Wait for the game to save. (can't manually save on Experimental)

5. Load the saved game.

6. The supplies are gone.

This also happens on pre-placed Arsenal services such as the ones in Montignac, Provins, Calvary Hill, etc...

NOTE: This bug is not new to Experimental. It has been a thing since 1.6. It's present in the current live version of the game as well (1.6.0.119)