Have you subscribed to a workshop item, played with it for a couple of hours, shut the game down, and later launched it to find that item does not appear in-game anymore? Does an item not appear in-game unless you start from the very beginning?

The solution here until a better one is implemented is to always begin by starting a New Game on Chapter 1, then loading a later save if you wish. You only need to load into the level, and not actually play it. This seems to work by loading the in-game environment without workshop content present in the level, allowing some internal check or overwrite to "get out of the way" so that Workshop Items will properly load-in when appropriate.

This seems related to when/how the Workshop Item assets are programmed to load in, and so this only applies to specific kinds of content. I will edit in a moment to provide two contrasting examples.

Example of problem + solution: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2785756234 This item only appears in-game if you load in without the MP5 spawned anywhere in the level, or by simply starting a new game before loading your save file.

Example of properly-implemented item: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2045182529 This item always replaces the base Glock without performing any tricks.