that is true. in fact i believe all the vanilla textures in the modding tools have that issue.
I meant that those folders are not contained on the released (published) version of the mod. we don't remove them manually. I tested this by publishing a version of our mod. our unpackaged version that contained the assets and assetsources folders did not have the thumbnail texture issue. but the published version that did not contain those folders and only contained the packaged assets does have the thumbnail texture.
The issue is still present on items with team colors. also, there are some vanilla items that are effected as well. The Issue only surfaces when the "assets" and "assetsources" folders are removed from the module (like your supposed to do when published). this makes me think the issue is with the compression/packaging of the assets when publishing the module (which is performed by the modding kit).
(edit) more information on the crash. it is related to village trade bound production due to the village location being closer to (or partially surrounded by) settlements from a different culture. the settlement distance limit of 150 that was introduced in the 1.8.0 stable (with no safe default) has been determined to be the cause.