I had like an interaction where I had some decent clues that a person only introduces their pronouns after they clocked me, e.g. other people didn't do pronoun intro before, the way the "ally in question" did it felt pretty artificial (I am willing to write that off on being paranoid).
The message is "I know you are trans, you are not passing, but I am graciously allowing you to exist in this group setting". Same goes for only doing pronoun checks in a group setting when you clock a trans person. I find it worse than assuming wrong pronouns and switching after being gently corrected. Maybe because I have higher standards of would-be allies than the average person. After all, allies are supposed to have my back, not just not having a 
moment when I pass by. On the other hand, I have it easy for being okay with being a semi-binary trans woman, so obv enby struggles are another thing, but clocking still stings like a bitch.
Pronoun checks should be normalized to a point cis people do it intuitively, but idk if we will ever get there. If you wanna chime in, when and how did you call that behaviour out or point it out, if you saw it? Did your response work?
What do you mean by this?
What makes this behaviour so uncomfortable is that it combines clocking, which is pretty uncomfortable, with being "nice", the pronoun intro. Even if the intent was to be nice to the trans person, it still pushes scrutiny on us and makes us feel isolated as well as making us question our presentation and appearance, which is especially hard for transfemmes, since bc of transmisogyny, we get judged for our looks thrice as hard as other people.
Thus what I receive from this introduction is not "This person is an ally", but "this person lords my appearance over me and wants to be my friend at the same time", which is incredibly patronizing and socially threatening.
Getting mad at someone for, by your own admission, trying to be nice sounds self defeating to me, but I hear what you are saying about it feeling patronizing.
To me it sounds like what you’re really upset about is being clocked. Being clocked is shitty. Clocking someone is awkward.
If you start the pronoun discussion yourself, does that help? Theoretically cis people should be doing that automatically, so if you don’t get clocked then great you’re normalizing pronouns. If you did get clocked then it changes nothing and it saves you feeling like someone is white knighting on your behalf?