It's not overflow. Han's bonus turn at the start is just that, a 100% TM bonus turn. Because they're tied at 100% turn meter, faster Han is supposed to win. It's right in the dev post regarding TM changes.
"Here is the order in which units will take turns if multiple units have reached at least 100% Turn Meter:
Unit with the highest Turn Meter overflow value
If multiple units are tied with the exact same Turn Meter overflow value, the unit with the highest Speed will act
If multiple units are tied with the exact same Turn Meter overflow value and the same Speed, one of them will be chosen randomly"
It was done that way to remove the coin flip that used to happen.