In your repository, I can see that the Agent has  something like this:

1:5.0.15-1+bullseye

 Unfortunately, Buster has:

1:5.0.15-1+buster

 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:

1:5.0.15-1+deb11

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.