For anyone else running in to this problem till this issue is fixed feel free to re-encode videos downloaded from the tiktok web interface, using

ffmpeg.exe -i DOWNLOADED_TIKTHOT.mp4 -c:v libx264 -preset fast -profile:v high -level 4.2 -pix_fmt yuv420p -crf 23 -codec:a aac -ar 44100 -b:a 128k -movflags faststart CONVERTED_TIKTHOT.mp4

And for a multi-thot post, quick and easy way to join many clips in to one meta video. Just place all clips in to one folder and run

for f in *.mp4 ; do echo file \'$f\' >> list.txt; done && ffmpeg -f concat -safe 0 -i list.txt -c:v libx264 -preset fast -profile:v high -level 4.2 -pix_fmt yuv420p -crf 23 -c:a aac -b:a 128k -movflags faststart output_video.mp4