Brep.Trim inconsistent behaviour

Hello,

I've been wracking my brain over this for a while now, and it works about 80-90% of the time, but there's something funny about it.

I'm trimming relatively large open Breps with smaller closed ones, effectively trying to achieve something like the BooleanSplit command, which is awesome because it works with open Breps. Basically, I'm splitting the closed Brep with the open one, keeping the bits that are opposite the normal of the open Brep, then trying to trim the open Brep with the closed one, after reversing its orientation so that it keeps what is inside the closed Brep, creating a cap for the closed, split one. 

Here are two closed Breps that intersect an open Brep:

Both closed Breps are completely intersected by the open one (no partial overlap). The result is the same with larger overlaps:

Here is the result. One works like a dream, the other gets the wrong cap after Brep.Trim. Both closed Breps are generated and oriented the same way, the open Brep is checked for normal consistency, etc.:

Any ideas? Any suggestions for how I could ensure that I get the right cap? I thought maybe using Split instead of Trim, and then somehow figuring out which is the cap might be the way to go, but I'm not sure how to implement checking which cap is the 'right' one... Any hints about the BooleanSplit command in Rhino and how that gets such consistent results? Perhaps I should try Houdini... ?