It must have something to do with the way you’ve set up your plugin.
To test, I just built what you described and it behaves properly. To start, I opened Motion and selected the Final Cut Title project option. Added the Scroll Text behavior to the text layer, and changed the text (content, font, outline, drop shadow just for kicks). Saved. Opened FCPX, applied the new Scrolling Text template over a generator background, replaced the text with several lines of gibberish, increased line spacing, alignment, other parameters…. still behaves properly.
Maybe crack that title open in Motion again and double-check everything.
I’m also running current Mojave / FCPX / Motion. Tested on a 2013 MacPro.
Doug Metz
Dalton Agency