Swing to Straight not working as expected

I am having trouble getting Swing playback to turn off as expected so I have set up a very simple project - minimal - to reproduce problem:

Test sample with single instrument and Tempo set to 2:1 swing 8ths (fixed) q = 100 using the tempo popover. Part being played simply consists of a series of quavers
Enabled Swing play back via Ctrl-Shift-P ; Default Rhythmic feel
5 bars later use Tempo Popover to change tempo to Straight (no swing) q = 100

On playback the whole piece is played in Swing; i.e. it fails to revert to Straight when it should

What am I doing wrong/missing?

When you reset are you doing “Straight (no swing) q=100” as a single entry in the popover? If so, that’s where you’re going wrong. First input Shift-T st - start typing and then use the arrow keys and Enter to select the precomposed “straight” entry from the list. This will fix playback and give you a hidden object (and a signpost if these are set to show). Then invoke the Shift-T popover again and type whatever you want to appear on the page.

I wondered what I was doing wrong, now I know