It ended up being security configuration on the server hosting the IPAs. I'm not entirely sure what the issue was, but moving them to a different server with TLS 1.3 fixed it.

I'm facing the same problem. None of our enterprise apps are able to be installed on iOS 14, it just pops up an error saying "unable to install, please try again later". Looking in the console shows an SSL error, despite the fact that the IPA is behind a valid certificate.