Flying mounts from Mythic Mounts such as the griffon should fall slowly while in the air, and do so normally when using Origins 1.1.2, but in both 1.1.3 and 1.1.4, they fall much faster, seemingly as fast as normal mobs (although they take no falling damage). This results in having to constantly press jump in order to regain altitude.

Still occurring in 7.0

Thanks for the report. I have looked into this, and can confirm this is definitely a bug.
Because Mythic Mounts don't have the source code openly available, I sadly was not able to pinpoint the exact cause of the problem.
I have created an issue on their issue tracker on CF. Let's hope they can resolve the issue or contact me so we can work together to fix it. :) Link: https://www.curseforge.com/minecraft/mc-mods/mythic-mounts/issues/62

Sorry for my delayed response and sorry for the inconvenience.

Can confirm this is still happening in Origins 1.6.1 and Mythic Mounts 7.0.