@certik @vsnyder
I found that information:
By default, on ubuntu 18.04 and newer the
add-apt-repositorywill also update the package index if the repository public key is imported.
https://linuxize.com/post/how-to-add-apt-repository-in-ubuntu/
Sorry, that’s why I did not report using apt update, but I will edit my post to add it. The behavior seems different on your Debian.