Not rare at all, if you listen for it. Only rare that it will create enough havoc to throw you off your game unless you run your IEM vocal very wet. If you're particular about your IEM bass tone, though, it's an issue.
This may a little more than rare, and it can be heard, but without some measuring software, it won't be easy to pin down exactly what's causing it.
It may be an issue for you, but I've not run into that to the point of feeling the need to address it. And I'm a retired sound guy who has deal with phase (the time vs frequncy type with lobes, nulls, and comb-filtering, not the electrical kind that is properly known as "polarity" and doesn't happen in the air).
And I didn't spend $3600 on wireless wear for my bass and my IEMs to not care about how my bass sounds. Same for the $700 I'm about to drop on a REDDI DI.
