It has to do with how zoomed out you are, and it is further exacerbated if you have a text style set to anything other than 1.0 for the text width. Reason being, it has to change that back to 1.0 when you "click in" to the text, which drives it all to hell based on the zoom. Its a known issue, that IS work-aroundable. Unfortunately, its by "butt-dyno..." What i mean by that is, you have to "just know" what zoom level to be at to edit the text. Once you know it, its not all that hard to work with, but its irritating.
We have one sheet that is an entire 30x42 inch drawing, of native Revit text. It isnt even in a Key Schedule. I can edit it without much issue, but you have to be used to it. The only reason its NOT done with key schedules is it was inherited on my arrival, and i dont mind working with it. Its a sheet of CYA stuff that no one will edit other than me, or i would redo it in key schedules.
Glad the KS are working for you Alex, ive been doing it that way for awhile with great results. I use whatever categories we dont need on the job.