Thanks for sharing the video.
If your final output is only a video - meaning your content won't be interactive - I strongly recommend you to use an AS3 document because it has much more features and less limitations.
You can easily convert your document to AS3 by going to:
File > Convert To > ActionScript 3.0
Please try this conversion and verify if the shape issue will still occur.
Thank you so much for replying, really appreciate it.
So I converted to AS3 (thanks for the tip) and exported the file again (mp4) and I still have the same issue.
I'd suggest two things.
Isolate the problem layer deleting everything else or better, copy/pasting that layer into a new document. Export to see how it behaves on its own.
If the problem is still present that would mean it is not happening because of some interaction with other objects.
Save the FLA with this one layer and share it by posting a link to it so that we can look into the actual shape.