Make absolutely sure you are runnng a Debug build.
Make sure you are in the plugin's Xcode project, the process you are attached to is the host, and you actually load the plugin in the host.
Create a file named .lldbinit (with the dot in front) in your home folder, and add this line to it:
settings set target.inline-breakpoint-strategy always
I am pretty sure I am using the debug build for debugging.
And I guess that wouldn't explain why it breaks somewhere and certainly not where my BP was. (Previous Xcode versions where picky about mixed line-endings and assigning the BP correctly, but that doesn't seem to be the cause here.)
Its not that some optimizations are turned on, creating a bit wired debug experience.