Tbh, it's not that surprising to me.

Trans prejudices are, as you said, rather new and to most people rather impactless. Most people don't have close daily contact with trans people, just because there aren't that many, so it's not that difficult to say "You know, I don't really care".

Man/woman stereotypes on the other hand are super deeply ingrained and even if you are conciously watching out for them and trying to avoid them you will inevitably fall prey to some of these. They are so ingrained that they are just subconcious.

For example, one of my kids had a birthday party. We were at some indoor playground and quite quickly the girls and boys of that group separated and each of the two groups did their own thing.

I checked in with the girls every 10 minutes or so, asking them if they are doing alright. After the third or fourth time I realized that I only did that with the girls, because apparently I subcontiously trusted the boys to manage on their own, but not the girls or something. So I stopped doing that.