If you try and select text of a previous post in order to quote, as you reach close to the left margin, the selection will JUMP and select all text previously on the page. The break point is right about where the avatar photo is being displayed. Tested on Chrome and Edge, broken on both.
Selecting text from right to left…
As soon as you reach just about the avatar image point, text selection jumps and the “QUOTE” tag disappears.
You can work around it by selecting with mouse to the broken point, then SHIFT-Left Cursor to include the rest of the text.