Don't want to use the term 'observed', nothing wrong with 'assigned'. As in, 'that's what was written down on the paperwork'. That's true.
But it wasn't assigned. Writing it down wasn't assigning it. It was a record of observing it.
Sometimes it makes sense to say trans or AFAB
It never makes sense to say AFAB because nobody ever had their sex assigned to them. It's not something that exists in a nebulous state until a doctor writes it down. It exists innately even if nobody knows it. It is not assigned. It never is. If I write down that a baby boy is a baby girl, I have not assigned him a sex, I have objectively, factually, recorded the wrong one.
You know all this, of course. If the words didn't matter, you'd use the right ones. It's precisely because they matter that you are so hell bent on using the wrong ones so you can obscure the truth.
This is why you "don't want" to use the word "observed". Because it's the right word and expresses the truth you think you can suppress.
Who, exactly, do you think you're fooling?