They can't and/or won't fix this. Anyhow, it's most likely out of miuiandroids hands.
The emoji works fine on aosp phones, like the galaxy nexus.. But NOT samsung (and seems HTC too?)
If you/we are lucky, xiaomi will fix this within this year, maybe not. Just don't get your hopes high though.. They have more important things to fix, i guess.
It would only get fixed ASAP, if the problem was on a xiaomi phone..
And just FYI. This is a framework thing, it's NOT just the mms.apk... So the jar files, probably framework apks too, needs to be modded for this to work.