I'm not sure why this merge would be producing the strange results I'm getting,-
I realize the object one extends past the starting point of object two and it fails,
but it fails in a strange way ...

The cylinder seam can be rotated 180 degrees...

- Brian

Hi Martin, boolean merge tries to intersect the objects and extract all solids that result but your objects here don't form any solids.

The main reason why it gets a weird result is that the lower object does not intersect the upper one in such a way that divides the upper one into 2 pieces, the intersection is more like a thin slit that does not divide the object.

MoI is mostly not set up to work on those kinds of slit or "strut" edges, you'll get some strange display meshes formed around them.

Generally the booleans are oriented around working with solid volumes and not combining open surfaces. It's usually better to use Edit > Trim to work on cutting up surfaces but again the cutting object needs to divide the base object into 2 separate pieces not only intersect it partially.

- Michael

Thank you, that explains it!

I've rotated the cylinder by 90 degrees and then performed the merge again,
I think these are the results I was expecting originally, instead of that weird outcome on the first try :)

Bryan: 180 degrees works as well, thank you.