Dora The issue shown in the video occurs either because you modified mesh vertices with the Deformed checkbox unchecked, as Spinebot pointed out, or because the bind pose changed from the setup pose. However, since you mentioned the mesh isn't bound to any bone, I understand that at least the latter isn't the case.
The remaining possibilities are that you might have accidentally moved mesh vertices while the Deformed checkbox was unchecked, or perhaps there are invalid vertices present. “Invalid vertices” refers to situations like vertices existing outside the mesh hull where they shouldn't be.
If invalid vertices are the cause, it's difficult for us to identify the problematic vertices without seeing your actual project. If possible, could you please email us a minimal reproducible project containing only the problematic mesh?: contact@esotericsoftware.com
Please include the URL of this forum thread in the email so we know the context. Then we can take a look at what's wrong.
