Same here, this is is crashing our site. I was able to promote a previous deployment from a few hours ago and that has at least brought the site back up.
Strangely, that deployment also use CLI 50.28.0 about 5 hours ago and succeeded. Everything after that suffered the issue discussed here - no middleware invocation whatsoever.
Also for future reference, I checked the Vercel status page frequently over the past hour and there was no mention of this - would have been nice to at least have some mention of an investigation instead of me debugging alone, diagnosing the issue, then doing a general search to find this community thread.