I think you might be running into a known bug: https://github.com/dotnet/maui/issues/17152

As per the latest comment it seems the decimal separator is taken from the device regional settings despite the localization of the application:

The numeric keyboard uses the decimal separator of the language as configured for the device. Not for the language of the app.
If the language of the device is set to english, the numeric keyboard allows the dot as decimal separator, even if the language of the app is set to dutch.
If the language of the device is set to dutch, the numeric keyboard allows the comma as decimal separator, even if the language of the app is set to english.
The numeric keyboard should at least use the language of the app as set in CultureInfo.CurrentUICulture to determine the decimal separator.