Okay, now that I knew what to look for, I found existing bug reports:

• https://lists.gnu.org/archive/html/groff/2023-04/msg00400.html
  
• https://savannah.gnu.org/bugs/?63960
  

Most importantly:

This is resolved in the groff trunk.

🥳

Yay, groff 1.24.0 is out and this is fixed now (on the left):

Image