Access token becomes invalid when access_token_revoked_at set specific values

Question

I have an issue in my resource protector in which everytime I do the access_token_revoked_at to have a value it always turn out that the Bearer token becomes invalid. However everytime I set it to default as 0 it becomes valid again. Im using oauth2-server in this:

class OAuth2TokenMixin(TokenMixin):
def default_access_token_revoked_at(self):
    return int(time.time()) + 900  # 900 seconds = 15 minutes

client_id = Column(String(48))
token_type = Column(String(40))
access_token = Column(String(255), unique=True, nullable=False)
refresh_token = Column(String(255), index=True)
scope = Column(Text, default='')
issued_at = Column(
    Integer, nullable=False, default=lambda: int(time.time())
)
access_token_revoked_at = Column(Integer, nullable=False, default=default_access_token_revoked_at)
refresh_token_revoked_at = Column(Integer, nullable=False, default=0)
expires_in = Column(Integer, nullable=False, default=0)