Sorry Emmanuel, i had not thought of you needing it, i deleted it to start over [:(].

If it helps.....
the only way i found that corrected some of the issue was to re-draw the walls, this however still meant that 1 corner still had the issue.

I concluded that walls joined in 6.4.0 were not happy opened in 6.4.2

I did recall that whilst the walls were not split, as you mentioned before, 1 of them had been cut/pasted, hence the need to join them.