I characterize it differently: the insertion point isn't visible at the left-hand margin of the reply window - I thought it was just subsumed by the vertical frame - and returns to blinking visibility after any character (including a space) moves it away from the edge. This occurs in the Quick Reply window as well as the Full Editor.

I think it's been going on for weeks, if not a couple of months; if so, it may antedate any software upgrade. I treat it as a characteristic, a minor nuisance rather than a malfunction, and ignore it.