I was struck by NR's hesitation when challenged to describe PL as a woman. He deflected, and spoke of courtesy/consideration. TIM often seem to think that people actually agree they're woman when people are only being polite.
I know two TIM, one is part of my social circle, the other is far closer. Neither pass. No one uses she or her when referring to them in their absence though we're all very polite to their faces. I imagine this is common.
Yes it is common practice, polite to the person, then reveal what is really thought when they aren't around. It's not just behaviour reserved for TIMs, it happens for many reasons.
A family member plays sport with a TIM, who remains on a male team, the men are polite to the TIM, then laugh at them behind their back. That is the reality of human nature and not only about transitioning people, it happens for many reasons.