So, I'd imagined as I posted that there might be some known setting I'd missed related to the 10 seconds. But now seeing one of my videos is 4 seconds but the other is 10 seconds. Given the coincidental 10 seconds, I'm going to get both videos to 16 seconds and see if that fixes my problem.
Issue is the same on two different machines running different versions of Cheetah3D and Mac OS (7.5.1 / 11.6 and 7.4.2 / 10.14.6 - updating a few things now that I see not everything is current).