Thank you for your trouble Mark. It was because I re-built my startup volume from scratch that I was rescuing templates from Motion 4 by opening them in Motion 5 and saving them as templates.
My thinking was 'what could be wrong with a simple circle?' Well, as simple as it is, it was problematic saving that Motion 4 template through Motion 5. No Drop Zones to publish first per one of your tutorials. Yet, it wouldn't work right. I mean, there's just one Group, and one Circle in the group. Nothing else.
When you reported that it worked for you, I deleted the circle and re-created it. Now it works fine. Thank you. But, what's the difference? I'm still at a loss as to why a simple circle drawing on would not translate. Chalk it up to the mysteries of graphic creation I guess. I've taken templates like a digital vu meter, an analog vu meter, and lower-third animations from M4 into M5 and thus into FCPX without a problem.
Now, I buit a simple arrow in Motion 5 and it animates in its length. I set a Mandatory Build-Out marker just after the animation completes.
Exported as a generator, it works fine. Publish a few parameters and save again and it no longer animates on. Unpublish all published parameters and it works again. Start adding back a published parameter at a time and it stopped animating at one point. I'm not naming that parameter because it's not consistently at fault. It appears to be some magic combination of publishing parameters in some magic order because now I'm back with a working arrow but the published parameters are the same, just enabled in a different order.