Custom Credit Roll does not scroll

In Motion 5.4.4, I created a title containing one Text item. I applied the "Scroll Text" behavior to this item and positioned it to begin below the bottom of the frame and scroll upward and off at the top. I entered prototype text in various sizes for a credit roll to be used in FCPX.

In FCPX 10.4.7 and 10.4.8 I apply this title to my timeline as a connected clip. I play that portion of the timeline. Sometimes the prototype text scrolls upward as expected; sometimes it displays the first "screenful" of text which does not scroll. When it does that, sometimes it will begin to scroll from that static position somewhere in the middle of the title clip, scrolling upward from there.

If it is scrolling properly, with this title clip selected in the timeline I change some of the text in Inspector -- for example, replacing prototype placeholders with names of talent and crew. I play the timeline again, and now the text no longer scrolls. This credit roll has become static text, showing only the first portion of the entire text.

Quitting and restarting FCPX often cures the problem. After a restart, the text scrolls upward as it should.

This problem has existed for several past releases of Motion and FCPX.

Any idea why this happens, and how to avoid or correct this?

Thanks!