I understand what it means when a tester shows "hot/neu reverse" in general: that the hot and neutral wires are attcached to the wrong screws.
I have a GFI outlet in a bathroom that, when in normal operation, the tester shows as CORRECT. However, when I trip the GFI, either with the test button on the outlet or the black button on my tester, instead of turning the power off (like other GFIs in my house), it shows "hot/neu reverse" on the tester.
I replaced the GFI with a new one and it still shows this problem.
What can I do myself to triage this situation, before resorting to hiring an electrician?