Wow, so oblivious to the problem...
It is absolutely a general issue. The reason you saw mods that don't have a problem with the thumbnails is that those mods - incorrectly - included the Assets and AssetSources in their download files...
This was reported in May and Taleworlds doesn't even take it seriously.
Now with your full release and modding kit publishing removing Assets and AssetSources forcing mod makers to correctly publish their mods, ALL mods with custom assets have this problem universally.
It is very easy to reproduce, the fact that TW hasn't acknowledged this bug even exists shows that TW never even tried.
- Make a new empty module
- Import any item
- Publish the mod
- Play the published mod, open inventory with cheat activated so you can see the thumbnail
- Observe how it has the stripy look 10/10 times
- Now manually copy over the Assets and AssetSources from the source module to the published module and run it again
- Observe how the striped look is gone
It is most definitely a bug with the thumbnail renderer.