You can fix that issue as the asset creator. What's needed is if you have your own copy of that prop stored locally on your computer, you can upload it and make sure the asset's using your version of the prop. That fixes the error and your asset looks how you intended it to. And as long as you don't yourself take that prop down, it that prop at least, will never break again.

Aha. The person who made and published the flag took it off of the workshop. I can't do anything about that. When creators take a prop, tree or other dependency asset off the workshop, all the other assets that use that item also have that item missing. The good news? A missing prop or tree from an asset will not cause you any problems. Yes, it will show up on LSM but it doesn't cause any problems with the game.

This mod shows a missing asset linked from it. The asset is "US Flag" which has an invalid link at https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=465387994 and pops an error for your mod missing the asset on the Loading Screen Mod.