Can't joint these two surfaces

Can’t see any reason why right now. I know it’s a larger red surface that’s culprit.
And you will be able to join them in the export file, even though I haven’t touched them after exporting.

unable to joint surfaces.3dm (55.2 KB)

What command shows the larger red surfaces as having 5 edges even though it appears to have 4 edges. That is the cause of the inability to join. The extra edge appears to be the result of the width of the trimmed surface vanishing at one corner. RebuildEdges does not fix the problem.

To fix:
DupBorder the trimmed surface
Untrim the trimmed edges
Trim using the duplicated border.
Join