In your repository, I can see that the Agent has something like this:
Unfortunately, Buster has:
Therefore, the Buster package is at a higher version than the Bullseye package, and upgrading a system from Buster to Bullseye will NOT trigger the upgrade of the zabbix-agent package.
Please use a scheme using the Debian release number instead, for example:
You'll notice that "deb11" is higher than both "bullseye" and "buster", meaning that anyone using your repository will nicely upgrade to the newer versionning scheme.