The issue was that as soon as there were no scopes for conflicting identifiers (they are present in conditional instructions, but there is none in the part 'b' of the  expression 'a and then b'). However, to avoid this inconsistency, the conflict is now checked before checking scopes. So, the error is now reported even when there are no scopes altogether.
Fixed in rev#100690 of EiffelStudio 17.11 intermediate release.