Out-of-range values are kept, and the Flag membership is kept. This is the default for IntFlag:
KEEP
>>> from enum import Flag, KEEP, auto
>>> class KeepFlag(Flag, boundary=KEEP):
... RED = auto()
... GREEN = auto()
... BLUE = auto()
>>> KeepFlag(2**2 + 2**4)
<KeepFlag.BLUE|16: 20>