@gabix: IMO any such case should be reported. It worked, then it doesn’t work anymore. That’s it. No claims are required to be entitled to report.
(I realize that you are experienced person, so writing the following just for completeness:)
What should be noted is the correct expectations. When one reports to bugzilla, it is great contribution; yet to avoid grief, one needs to realize that closed bug report (e.g. WONTFIX or NOTABUG) is a valid result, and is not an offence. (Me personally has many issues reported, that were eventually closed like that; and of course, I closed quite a few others’ reports with those resolutions.) So it’s essential to be prepared that a report is closed that way, and not be depressed because of that, and continue filing bugs in positive way.