Not sure what's causing the bug (I can't reproduce it on my end) but one thing worth noting is that you're using outdated templates.
If you're using the latest version of Animate, try using these newer templates and see if that solves your issue: Creating HTML5 Ads with Animate CC: Google HTML5 Ad Templates | Adobe Animate Team Blog
You don't need to use AdHelper unless you really want to, and you definitly don't need to reference TweenMax in your HTML file since you're just using timeline animation.
Thank you for checking those and for pointing me to the latest templates. I just tried one and, though I can't tell if that bug is eliminated (since I can't reproduce it anyway!), it does seem to streamline the final output process. I will follow up with more on my results later. Thanks again!
I know, right?! I thought that "t" was going to be my big clue in diagnosing this problem, but I couldn't find the code that would've produced that.
In any case, so far, so good with just.emma's solution: using the new templates.