Actually, I stand corrected. I wasn't following your directions correctly. My apologies. I played with this a little more with 4k clips and I am experiencing the same issue with the PIP swap feature that you are. It doesn't malfunction with two lower res clips, but it does malfunction with two 4k clips and with one 4k clip paired with a lower res clip. So it probably is a bug with 4k as you suspected.
You might be able to split off the specific sections of the clips that you are working with, copy and paste them back in so you have two sets, add the PIP to the first set, and then reverse the timeline position of the second set of clips from top to bottom, and add in another PIP. Or work with 1080 clips for that specific section. It's still very hi def and might not even be noticeable.