So I was right to be paranoid
As a rule, the current Kobo ereaders use the same firmware. There are differences in the UI but they are running the same code. I am running a different firmware version but I would not have expected this to be something that has changed. Having said that, it wouldn't surprise me if the "<br>" caused a problem but the "<br />" was OK.
And I just realised why I didn't see the problem. I have "replace_br_with_p: true" in my personal.ini. The "<br>" was disappearing completely. When I removed the option, the test cases broke on my Glo. And stories created with the updated version of the plugin work with no problems.