My practice is to make all tags lowercase. As a result, the only “tag” that begins with an uppercase letter is “Untagged”.
I acknowledge how microscopic of an inconsistency this is, and I have no issue living with this limitation (if that is in fact what it is and it is there intentionally).
That said, please let me know if:
A) I am missing where this can be changed
B) If making the “Untagged” tag capitalization as a preference (“make all tags lowercase” checkbox?) could be added as a feature request?