You may or may not be aware that there are generally two versions of videos available on TikTok's servers, the one with a logo superimposed on it that you generally see, and a somewhat higher-quality logoless version that you usually have to use a site like tikdown.org, snaptik.app, or ssstik.io to get. I believe the logoless version is the actual originally-uploaded file, or at least closer to it, and generally recommended for upload to the site (TikTok has to recompress the videos to add their logo, which makes them look worse).
Recently I noticed that the logoless versions of some videos only seem to be available as higher-resolution HEVC/H.265-encoded MP4s. The actual container format seems to be closer to Quicktime .mov's, but .mov and .mp4 are basically the same thing plus or minus a few bells and whistles anyway. The real issue is the video encoding. GWM doesn't allow uploading HEVC videos, for good reason: basically no browsers support them outside of Safari.
These new higher-quality logoless versions (we'll call them the 'HD' versions as some of the download sites seem to be doing) seem to generally be 720x1280 or 1080×1920 (the maximum upload resolution TikTok allows), rather than the 576x1024 that was standard for both the logo'd and logoless versions previously. I can confirm that the HD versions seem to be coming from TikTok's servers, since I'm able to download the same versions directly with a browser extension in some cases. They also seem to be genuinely better-quality originals and not upscales, as far as I can tell.
Theoretically the HD versions can be converted to standard AVC/H.264 MP4's that GWM supports, but that's both a pain, and in a lot of cases would balloon the filesize past the 25MB limit if you have the encoding quality set high enough to maintain the better look.
I should note that this isn't universal for all videos. I first noticed this with Brissa Allen's TikTok. This one for example has the HEVC HD version available without a logo while the 576x1024 AVC version still has the logo on it, no matter which download site you use. This one has both standard and HD logoless versions available, if you use one of the sites that lets you choose. This one seems to have only the HD version available, regardless of which option you pick.
I'm glad higher-res versions are available, I just wish TikTok had stuck with AVC for the encoding. I imagine they're trying to save on server space and bandwidth since HEVC gives smaller filesizes. There are some other interesting technical details, but the upshot is that none of the HD versions can be uploaded to GWM without performing a lossy conversion on them, and some videos won't have usable versions available using a lot of download methods at all, without needed to be converted. I'm also going to be switching back to using snaptik for downloading myself (I had been using tikdown), since it lets you choose between versions, which is more important now.