Equivalent stress above acceptable levels, but check remains OK. Why?

The checks follow the EN1993-1-5 rules. These are summarized in the following figure taken from Figure C.2 of Annex C of EN 1993-1-5. As a natural outcome, when the solution is in the plastic branch (which is the usual case for steel) the stress is slightly higher than the yield stress due to the inclined top branch of the working diagram.

Please note: The ultimate criterion is the strain limit. If your model satisfies this limitation you don’t need to check stresses. They are presented for the sake of completeness.