Got the new public beta today and that fixed the scrubbing issues, but Safari is now running my CPUs up and getting hot. But the Safari Technology Preview is not using high CPU so I'll stick with this for awhile.
Yeah, Dev Betas 1 & 2 made YouTube unbearable for me. Constant lag and scrubbing would cease during videos. Safari Tech Preview fixed those issues for me then, but the 3rd Beta removed any scrubbing issues I had prior when using regular Safari.