Hi @trois ,
Are you perhaps having an issue with flipped normals? This can make the visual information on the canvas very confusing.
To check, go to Tool> Display Properties and make sure that double is disabled. Now the mesh will only appear when the normals are facing the viewer. If you see large gaps of empty canvas now where polygons are expected, this may indicate that the normals are facing the wrong direction. This can be corrected with Display properties > Flip for the affected geometry.
If there is no display issue with your normals, are you certain that both parts of the mesh are actually rotating? Or could it be the case that one portion is static while the other is rotating? There could be many explanations for this.
Ultimately the geometry in the file could be problematic or corrupt in some way, but I don’t usually see issues in the way you’re describing. If that is the case then you would need to correct the problem with the file.