I'm running into this too. It happens when adding text to an existing comment, not just deleting, and only when editing multiple characters at once (swipe typing or holding backspace). It doesn't seem to happen when pressing a single button at a time.
The bug only happens once and seems to trigger the first time the number of lines changes and Voyager has to rewrap the input text, but it's not consistent (I've edited this comment about a dozen times now without it happening). And it never happens on a fresh comment, only when I go back and edit one later.
Running on Android with the HeliBoard keyboard.