BUG 5.1 RC1: Selected text in box isn't deleted/replaced

I have seen this all day, and in many places. I thought I had finally found a repeatable case -- but, of course, as soon as I'd written it down, it wasn't repeatable any more.[:'(]

What happens is this: I select the text in a reference box of some sort (mostly it's been in BWS). I then type in the first letter of a new word, or hit Delete, at which point the old text comes back, deselected. Or the new text gets added to the end or in the middle of the old text. Very annoying.

Anyone else who's seen this?