TYPO3 Core

I noticed something with one of our banner elements where we use Mask, found a 'workaround' and I'm not sure if that is a bug. I created the issue on the mask github first (https://github.com/Gernott/mask/issues/442) but was told that this is probably a TYPO3 bug so I'm open an issue here now.

I have a banner element with 3 slides (German, dummy language which is disabled in FE) and wanted to localize the banner (English).

(see "German slides in BE" image)

Then I localized the banner element.

(see "English localized slides in BE" image)

36 => 816
137 => 817
795 => 818

It shows the German records 136 and 137 twice. One time localized and the other one is disabled. There is the "Alle Datensätze lokalisieren" Button but when I click it, nothing happens. I assume that's because there are non localized elements shown which were already localized and therefore the button has no function.

This seems to only effect old IRRE elements. The slide "Test" (795) was added later (under a newer Mask version) and this one is not available a second time.

I looked in the DB and found this differences:

(see "DB" image)

It seems that sys_language_uid is -1 for both older slides which appear twice.
Also, l10n_parent is 0 for the new localized slides but it is available for the new "Test" slide.

If I change sys_language_uid to 0 (in the BE) the slides don't appear a second time (after a new localization).
The "Alle Datensätze lokalisieren" button is also not available anymore because everything was localized.

It seems to happen with elements which have the language id -1 ("Alle").

My question: Do I have to set the sys_language_uid for older IRRE elements (maybe other elements aswell) to 0 so that those elements won't appear a second time or is this a TYPO3 bug?

Currently I us Mask 7.0.27 and TYPO3 10.4.21.