I wonder why the hyphenation fix for the Plone toolbar is not part of the 5.1.0 release?
Because you pointed your PR to master, which was 5.2 by then? Please work carefully.
Right, thanks for the pointer but how do you get such a change both in master and the 5.1 branch at that time? Two PR?
Yes, two PRs would be needed.
Sometimes a reviewer will mention this.
Sometimes a reviewer may do the work him/herself, but usually not: there is always enough other stuff to do. And on a small PR the extra work may be small, but also for a reviewer it would still involve creating a new PR and running the tests.
That's one way. I am not sure but I believe if you had committed in the 5.1 branch that would/could be rebased on top of more advanced branches, like master (5.2).