Hiya, this seems to be a problem with it being moderated rather than the website being bugged. Are other users able to wear the shirt and see that they’re wearing the shirt? If not, I’d suggest making a post in #moderation-review-requests, since the shirt was likely incorrectly moderated.
I’ve had this issue in the past, usually I just try to re-upload it a few more times until it eventually works because the thumbnail is vital if you want people to buy the asset.