I think that is a bug I have had in Sierra and still (I think) inMojave with the latest RW,
My workaround is to type the text in another word processor (or TextEdit) and paste it into the field.
It’s been noted before on this forum.
On second thought, I think I noticed that problem first when typing in the metatag decription field. Something is happening but I’m not sure when it started, for me. I’ve just gotten used to accepting some odd behavior since RW 8.03 …
@dan - it looks like what’s happening is that changes in the field are “continuous” – so each change is noticed and triggers the preview to re-render. that re-render is calling “finish editing” on some views (or something similar) – so the insertion point gets stolen while the user is still editing.