Toggling message headers has no effect on the current message

I noticed that I cannot change the message headers anymore. Meaning that selecting View > Headers > All (or Normal) has no effect on the current message. Opening a message in a new tab shows it with the current setting.

When disabling the Allow HTML Temp extension it works as expected (changes the headers in the current shown message as well).

Is there anything in the extension that would prevent this? Can someone else confirm the same issue?