That's strange, which version are you running?
Check the git tags, that tags are not being reported

I did some further investigations. I wonder if this is causes by the difference between complete new changelog and incremental.
If doingcz changelog, then it is implicitly done as a complete new changelog, correct ?
If I do acz bump withupdate_changelog_on_bump = true (as I do) then this implicitly done as incremental changelog, correct?
This would explain the behaviour as described.

However, I think it is desirable to fix that to allow to maintain a headline in the template without running into the trouble this headline to be duplicated during incremental changelog updates.

I think I am right :-)

To overcome I guess an easy improvement would be to provide the changleog generation mode as additional variable. This would allow to control in the jinja template if an headline is generated or not. For incremental changlogs then the headerlines (or footer lines if desired) would not be generated.

Another option:changelog_incremental is not automatically set to true forcz bump.

So, tried it.

One additional line (inserted between lines 247 and 248 in /commitizen/commands/changelog.py

        changelog_out = changelog.render_changelog(            tree,            self.cz.template_loader,            self.template,            incremental=self.incremental,   # <-- inserted line here            **{                **self.cz.template_extras,                **self.config.settings["extras"],                **self.extras,            },        ).lstrip("\n")

... and following change in the template

{% if not incremental %}# CHANGELOG{% endif %}{#  MACRO: Capitalize the first letter of a string only #}...

does the job.