I did compare video files between the old endpoint before and after tiktok made changes and the new experimental endpoint
This is all same file before and now:

Old endpoint before tiktok changes:

Video:
Frame width: 720
Frame height: 1280
Data rate: 4614 kbit/s
Total bitrate: 4740 kbit/s
Frame rate: 31.06 frames/second

Audio:
Bit rate: 125 kbit/s
Channels: 2 (stereo)
Audio sample rate: 44.100 kHz

Old endpoint after tiktok changes:

Video:
Frame width: 720
Frame height: 1280
Data rate: 699 kbit/s
Total bitrate: 763 kbit/s
Frame rate: 30.00 frames/second

Audio:
Bit rate: 64 kbit/s
Channels: 1 (mono)
Audio sample rate: 22.050 kHz

New experimental endpoint:

Video:
Frame width: 576
Frame height: 1024
Data rate: 974 kbit/s
Total bitrate: 1006 kbit/s
Frame rate: 30.00 frames/second

Audio:
Bit rate: 32 kbit/s
Channels: 1 (mono)
Audio sample rate: 22.050 kHz

Edit;
After comparing about 50 files i can say that some files are better between new experimental endpoint and old endpoint after tiktok changes. Some are unaffected and some are worse. In many older videos files new experimental endpoint gives you lower video resolution but slightly better quality.