Advanced layout editor changes content in text block unintentionally

Hi,
I have a textblock incorporating an iframe. The src param contains a variable ‘ampm’, which is responsible for clock display 12/24 h.
So my code looks like

<iframe class="timer" src="https://webuhr.de/embed/timer/#date=2021-05-13T19:55:00&ampm=0&theme=0&m=0&color=3&m=0&sound=pizzicato" width="1280" height="auto" frameborder="0" allowfullscreen="allowfullscreen"></iframe>

Full content of the text box is given as private content.

When i click on [Save], all is good: I get 24h clock. Now I publish the page, still all good.
Now I load the page again in the editor. Preview in ALB still looks good!
Now I edit the text block again – boom there we are! My iframe has changed to

<iframe class="timer" src="https://webuhr.de/embed/timer/#date=2021-05-13T19:55:00&m=0&theme=0&m=0&color=3&m=0&sound=pizzicato" width="1280" height="auto" frameborder="0" allowfullscreen="allowfullscreen"></iframe>

As you can see, my ampm-param. has turned into another m-parameter (so there are 2 now).

What’s going on here? Is it a bug? Or how to avoid that?

Best!
Jürgen