Invalid certificate coming from fastli

I keep seeing security errors and pages not loading from Gitlab Chrome reports an invalid certificate.
When I check, I always see something in fastly as domain name.
Reloading, force-reloading does not work.

Example of URLs that show this security problem :

Using a Dell XPS 2020 Developer Edition
Ubuntu 20.04 (machine came preloaded with 18.04 and did the 20.04 upgrade)
All software up to date according to apt. Latest kernel, yadda, yadda, yadda.

After waiting 5 minutes or so, reloading the page gave me a valid certificate for

Why am I receiving certificates from fastly when accessing Gitlab ?
And especially, invalid certificates ?

Example : Screenshot-from-2021-01-27-02-54-18 — ImgBB
Someimes the name changes to something like (with or without the global part)
When I test I can see that the domain itself does not exist…