So I discovered this this morning...
I shot an event on Saturday, taking my X-T3 and X-T4 and some fast Fuji glass. Turned out the 16MM f1.4 lens would not auto focus on the X-T3 though it was fine on the X-T4.
Well, today I learned that this was my own fault - I had inadvertently pulled the focus ring backwards to disengage the auto focus clutch, so that answered the question as to why it didn't auto focus on the X-T3. And I have to thank the tech at Fuji - I called this morning to ask about this and he asked if the focus ring was disengaged...
But - and here is why i am posting this here - that same lens set the same way DOES auto focus on the X-T4! It's as if the X-T4 body just ignores the setting on the lens - and that was something the tech (and the other ones he asked about it) never had seen before.
So those of you with that combo - give it a try - it's just odd.