Just read some of the new rules, two days to clear for asymptomatic vaccinated players. But the new rules also specify testing of vaccinated players only if they are symptomatic or a close contact. So he might have been tested due to symptoms.
If a player is symptomatic they can't test out. They need a minimum of five days and negative test...
We don't know if Mills is symptomatic. But Bryant was, and he was out for 8 days.