Operating system: Linux (NixOS)
Graphics card: Intel
Short description of error
Smart UV mapping seems to work significantly worse (at least for my test case) since the C rewrite, which landed in 2.91.2 (I think).
Exact steps for others to reproduce the error
Perform Smart UV mapping on the mesh, as detailed in this video: https://www.youtube.com/watch?v=illIxYKb-1k