When the timeline is in Insert mode, Magnetic on, and you drag the left edge of a clip to shorten(Move Mouse right while holding left button down) it, you still get random 1 frame clips of the clip to the left of the clip being shortened, of the clip to the left of the clip being shortened.

I.E. You have clips 1, 2, 3 in a row on the timeline.  You drag the left edge of clip 2 to the right to shorten the starting position, you randomly get a single frame(Looks like last frame) from clip 1 inserted in between  clips 2 & 3.

It doesn't do it every time.  Working on a 10 minute video with about 70 clips, and it has done it twice.

Source clips are 4K 60fps progressive.