Hi Stefan,
Greetings from Syncfusion support.
We would like to let you know that, Html Attribute is working fine at our end. But if we provide type as tel for TextBox component it is working fine in other platforms, but type value not override in the C#. So, we have confirmed the reported issue as a bug at our end and this fix will be included in the first patch release after Volume 1 SP1 release which is expected to be rolled out on the 5th February 2020.
Regards,
Sureshkumar P