Hi and thanks for the quick reply. I appreciate your explanations as understanding the reasons
behind the problems helps me avoiding them in similar situations.

No patch tp patch was involved here - I rotated a curve and cut out a chunk with boolean op.

"either side of that edge are not quite smooth to each other"

Possible that the initial curve I used was nor perfectly smooth . I attached it below.

"Hi Tom, also what radius value are you hoping to use there?"

I wanted to try out fillets of different radius and then choose one if them