Sounds like a VE issue. Moving there.

If this happens in FF, can it be related to bug 54917?

(In reply to Elitre from comment #2)

If this happens in FF, can it be related to bug 54917?

Doesn't look like it; this was caused by insertion, not cursoring.

This happens just by typing anything after an inline focusable node after cursoring off it (nothing to do with deletion). If the node is at the end of a paragraph you get a pawn, but if it isn't you get equally broken behaviour with the cursor not moving properly.

Monitoring document.activeElement (the currently focussed element) it's apparent that simply setting a selection on the document (back from the paste target) doesn't changed the focussed element. This means you can move your cursor in one textbox and type in another. What the hell Firefox...

Change 145346 had a related patch set uploaded by Esanders:
Make sure the document is focused when setting a selection

https://gerrit.wikimedia.org/r/145346

Upstream: https://bugzilla.mozilla.org/show_bug.cgi?id=1037004

Change 145346 merged by jenkins-bot:
Make sure the document is focused when setting a selection

https://gerrit.wikimedia.org/r/145346