I am using 2018 R4 in OS X 10.11.6 (yes, I know, that is another conversation). I have been using this release for the past few weeks and breakpoints were working fine until yesterday. I have restarted both Xojo and my computer and neither step solved the problem. The break command does work. I searched and saw this was a bug in 2018 R1 but saw no posts about breakpoints being broken since then. I will be switching from this old Mac to a Windows/Linux system soon, and can use the break command, so I am just posting this here in case anyone else finds it useful or has anything to say about it. I will report this via Feedback as well.
I have not experimented to find out if all breakpoints are not working or just certain cases.