People Picker Limitation Problem

I have a nice Problem with the People Picker Field on SharePoint Online:

1. I have a Modern Teamsite without a M365 Group
2. I have a List with a Content Type that has a People Picker Field, allows multiple selections, Only People are selectable that has to be chosen from the SharePoint Visitor Group (or any other created on that Site Collection)
3. When i try to add more then one Guest Account (by Email), i got the error that external sharing is disabled, but external sharing is allowed and works fine.
4. The same behavior when using grid View of the List
5. When the User is found and selectable, after selecting the field doesn't saved the found account
6. Another Result (when using UPN or Claim of the Guest) We couldn't find an exact Match -> Guest was invited to this site and has access, is also in AAD listed as Guest.
7. Using Power Automate adding guest accounts to the same Field in the List works!

I realy need the Limitation because of GDPDR restrictions, but it doesn't look like working correctly.

Any Ideas what could be wrong, special settings i have forget to set within the SharePoint Group or anything else that must be done before?

Thanks for Help