Install instructions failing with signature warning

I am trying to follow instructions HERE and I am consistently getting an invalid signature warning:

W: GPG error: MongoDB Repositories buster/mongodb-org/6.0 Release: The following signatures were invalid: BADSIG 6A26B1AE64C3C388 MongoDB 6.0 Release Signing Key
E: The repository ‘MongoDB Repositories buster/mongodb-org/6.0 Release’ is not signed.
N: Updating from such a repository can’t be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

I can still successfully do a sudo apt-get update for version 5, but 6 fails. Should this still actually work for version 6?

I have done sudo apt-get clean, and also gpg tells me the key is valid:
sudo gpg --show-keys /usr/share/keyrings/mongodb-server-6.0.gpg
pub rsa4096 2022-02-23 [SC] [expires: 2027-02-22]
uid MongoDB 6.0 Release Signing Key

Am I missing something simple?

…and then yesterday, just like that, it started working.
Definitely something within our testing environment.