For what it’s worth, this is happening to me on multiple sites, on different physical servers with different themes and plugins on each.
I’ve also tried in browsers with zero blocking on it, just in case it was something on my end (which would be logical, given the circumstances.)
Clearly something in the latest release isn’t compatible. Is there any sort of logging type ability that could shed light on the issue? I don’t relish the idea of going over 4+ sites to find the issue.