I am having the same issue. When loading into an editor session, just as the session has almost launched all loot chests are being instantly deleted in game and UEFN (they are removed from the outliner and checking in shows them as being removed). This is applying to to all props from the Chest and Ammo Galleries for me.
Platform. Windows
Update: Also occurs with custom chests created from with the session (creative), and with the Fishing Rod Barrel prop.
Examples from output log:
LogActorLevelEditorSelection: Warning: SelectActor: The requested operation could not be completed because the actor has invalid flags. (Fishing Rod Barrel6) Flags: (0x00200001)
LogActorLevelEditorSelection: Warning: SelectActor: The requested operation could not be completed because the actor has invalid flags. (LootChest_9) Flags: (0x00200001)