Seen this before and experienced the exact opposite.
I’m mid tier 10-14s with the 1450ish IO to match. I have joined more than one 2 or 3key a lower tier player posted just I could get the easy valor. Smooth run.
I have also tried my hand at tanking a 5, and we had a 3000+ player join us knowing I was a green tank. Not so smooth run but the guy had a blast, hung out and talked a bit after the run.
Best advice I can give is only try to join groups at or just above your current standing. Avoid “must make timer”, those have toxic potential.
When posting your key, let them know about the situation, “new tank”, “carrying a friend” or “may not time”. This should attract patient players. Just be honest with your objective.