When not under the influence of the YAM AURA. Mongols Handcannons are only receiving ~11.1% bonus movement speed (in-game 1.25). As opposed to every other civs handcannons which get the proper 20% at 1.35 in-game.
When under the influence of the Yam Aura they are receiving the correct amount of move speed.
I cannot test for if this is just a visual UI bug but I would be surprised if that as the case. I am not aware of a precedent for that occurring.