Cynthia didn't lose, Garchomp deliberately jobbed.
She didn't want Cynthia to retire and knew what was at stake had she thrashed Ash and then Leon effortlessly.
Garchomp cared for Cynthia's wellbeing and losing this battle showed Cynthia that there was still so much more to do and accomplish as the Sinnoh champion.
Thank you based Garchomp.