Files are building/playing differently in Vegas Pro 20 than in Vegas Pro 18

NOTE: For some reason this was flagged as spam when I first posted it. The only thing I can think of is that the bots didn't realize I answered all the required questions in the post, so I will put those in boldface to see if that makes a difference.

Hi. I am currently switching from Vegas Pro 18 to Vegas Pro 20, as well as from an old laptop to a new(er) one. I will list specs at the end of this post, but it seems more likely to be a software issue (legit copies).

I have discovered that mp4s that I have used just fine in Vegas 18 are behaving differently in Vegas 20.

Nuts and bolts of the project: I am rendering out several shorter mp4 "items", that, after the client's approval, are assembled into "full" projects and rendered as such. So, yes I am re-rendering mp4s into larger mp4s, which I did not think was a good idea at first, but which seems to be working fine. I even tried some "copy of a copy" experiments by re-rendering files in several generations, with zero visible or audible artifacts. Anyway, this method has been working adequately for years now starting in Vegas Pro 16 and then VP 18.

The issue: I recently upgraded to Vegas Pro 20, first on the old laptop, which now houses both 18 and 20, and then on the new one, which only had Vegas 20 at first, but now also has VP 18. Now, when I try to use previously rendered mp4s, some of which have been used in multiple previous projects with no issues for months or even years, they now have a small extra blip of sound at the very end of the file. It's only about .25 secs, so I can't identify it, but it is not random noise. It sounds like it might be from within the file itself, or possibly from another one. If you zoom in on the waveform, you can see it there.

Now, here's the really weird part: if you open that exact file in Vegas 18, it behaves normally and the sound is not there. If you open it in V20, it is, waveform and all. This is true whether I use V20 on either my old laptop or the new one. I even tried putting it on an SD card, to eliminate the possibility of corruption during copying, and the same thing happened with both programs, on both computers, pulling from the same exact file. I opened the file in Audition and it was fine. I even checked it in Reaper; totally normal. I am only referring to one file here, but it is not the only one affected. However, it seems that not all the mp4s in the projects are affected.

I have never heard of or encountered such a thing before, and I have been unable to find someone having quite the same issue anywhere via Google, Reddit, or anywhere else..

Any help is much appreciated. Thank you.

SPECS (Programs, OS, and BIOS all fully updated):

Old laptop: Dell Precision M6600

Windows 10 Pro 64 bit, build 19044.2604

Intel i7-2860QM @ 2.5GHz

NVIDIA Quadro 3000M

16 GB RAM

New Laptop: Dell Precision 7730

Windows 11 Pro 64 bit, build 22621.1265

Intel i5-8400H @ 2.5GHz

NVIDIA Quadro P3200

64 GB RAM