@KarstenI is right! Some other Vendors do that kind of configuration as well. And it kind of makes sense if you consider that "Native" VLAN is UNtagged.
So in that FMC interface actually implies that all subinterfaces are tagged and the only UNtagged would be the physical. By the way, allowing UNtagged isn't the rule but the exception. Usually, you wouldn't route untagged traffic.
If you're curious, check more here in this Cisco Doc.
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