Input mode, keyboad input adds chord to wrong note

In the attached snippet you'll two bars, the first is what actually happens when I try to type what I want, which is the second bar.
The sequence is this:
4/4 empty bar
enter mode (N)
5 0 (crotchet rest)
4 0 (quaver rest)
C#-G-C chord from the usb keyboard
6 0 minim rest
Ctrl-3 (triplet of crotchets over then next minim)
back arrow over chord
+ (crotchet is still selected, tie to 1st crotchet of triplet and automatically enter the chord)
D-F#-B chord from keyboard. Second triplet chord is entered, cursor advances
(all is good at up to this point, one triplet rest left in the bar with the cursor over that triplet)
Db-G-C chord from keyboard
the notes are not added to the cursor position but instead to the previous chord, which is now a 6-note chord.
I now have to use Ctrl-Z three times to undo the added notes and replay the Db-G-C chord.

This happens very frequently. I've never been able to pin down the exact requirements. A tie into a tuplet seems to trigger it.
I'm on MU 3.6.2 under Ubuntu 22.04 - this is not new. I've seen it for years going back to quite early releases of MU and Ubuntu.

Richard