I have the same issue - can’t edit longer texts because can’t place cursor in higher sections of text.
I’ve been up to date with 17.1.1 pretty much since it came out and the issue persists. None of the fudges recommended in this post work. Including the most recent of dragging up from the space bar - keyboard goes grey yes, but cursor disappears and also still can’t scroll up. I booked an appointment with a genius and he tried this and failed. He tried various other things too, to no avail.
In addition to the scrolling up / editing issue in the messages app, the curser doesn’t function as it did in previous iOS in other ways as well. It is much poorer. For example when I tap to a position I wish to edit, sometimes it goes to where I want to place it, while very often it selects a word or even letter, and for the life of me I can’t just get it to flash in the spot I want without it selecting something. Whereas this wasn’t a problem in previous iOS. (It’s happening now while I write this, not in messages.) Then I tap in a different place and it does as one wishes and is the normal flashing cursor again. But it’s not where one wants it. I can’t see any pattern to it. It is infuriating for sure.
Everything is taking longer.
Getting the user interface right / having a curser working properly is obviously an absolute fundamental.
This is really poor and it’s been happening for way too long now.