This occurs in V5 - V7.11:
Directly deleting entire rows of control points from a non-rational surface converts it to rational even though _List reports all weights as 1.
Yeah, I’ve encountered this before, from a different cause though - I can’t remember the reason, I asked at the time but… let me ask again…
RH-66021 Removing surface control points makes it rational
-Pascal
https://mcneel.myjetbrains.com/youtrack/issue/RH-61777 was about _RemoveKnot creating rational curves, but not surfaces. Directly deleting control points makes surfaces rational, but not curves.