I made a simple sketch in an assembly. Four lines with two co-linear relations.

Later some of the assembly geometry changed and the co-linear edges no longer existed

so I received the expected error with the sketch "geometry no longer exists".

I deleted the relations and made new ones. The error persisted.

I rebuilt, the error persisted.

I saved, closed, and restarted SW, the error persisted.

I edited the sketch, deleting all four lines and drawing them new, with new relations. The error persisted!

Just to be sure there were no extra lines, I selected all, Ctrl-A, and it confirmed only four lines (all new now).

Note: the sketch plane is not the error. SW is saying it is something in the sketch but showing nothing wrong.

The question is, is there anything I might have missed? Or is it simply a bug / corrupt sketch that will never work again?