Wrong content shown in in Panel inside Multilang Structure Field

[GB_DESIGN]

That doesn’t look normal.
I would test this with a fresh starter kit to rule out that it is a Kirby error.
Then you can also use the new RC2:

Kirby 4.0.0-rc.2 is here!

v4 is getting closer. Today we are releasing RC.2 https://github.com/getkirby/kirby/releases/tag/4.0.0-rc.2 As you can see in the release notes, we are not making massive changes anymore, but focus on last patches. We’ve also received a lot of new translations since our call for translators on Discord yesterday, which is amazing! If you still want to join, please sign up as translator on Transifex: https://app.transifex.com/getkirby/panel/v4/