[Edit: wrong link.]
I may have stumbled upon a possible bug, or at least a quirk. Sometimes the merge tag button (not the other merge buttons) does not work. (My guess is that) the merge tags button does not work if the left version is entirely contained in the right version...i.e. when a copy tags would work just as well...though this is of course a sort of workaround, it is cumbersome to use in that way (with many tags, and depending on the order, it is not always possible on inspection to confirm the condition). The merge button should still work IMHO.
video of it in action:
https://imgur.com/a/mC5bhOC [corrected]