V7
V8
V9
MAF.3dm (620.1 KB)
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sys info.txt (3.5 KB)
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hi @Piotr thanks for reporting,
I logged:
RH-83072 MergeAllCoplanarFaces is broken in Rhino 9 WIP
ps: the object is an invalid Brep, but that is not why it is failing.
I thought it may be invalid by SelBad finds nothing.
SelBadObjects works here in 7 8 and 9 to flag that object.
?
It is valid here: 
interesting… are you sure you’re looking at the same file you posted?
I’ve sent an V7export after copy-pasting it from V9.
Strangely, it is valid in V9 on the whole scene and becomes invalid when opened solo.
Could this be related to the mess that results in V8 and V9 when Rebuild is used on a single surface in a polysurface? Rebuild and sub-objects
I don’t remember how it was created but, for sure, Rebuild wasn’t involved.
My thought was there may be some common code.