The sub property appearing to change for the same email address is a valid scenario. SPs failing to respect that scenario because they don't understand it, or because it's not what some of their users want, is not a valid excuse.

https://support.google.com/a/answer/33314?hl=en&co=DASHER._F...

To me it is reasonable that orgs may want to eventually reuse an email address on a different user account. That's a feature decision made by the IdP so SPs need to respect it. I believe other IdPs like Okta and Entra have equivalent features too.