Hi! I'm on a Mi 8 Lite with the official global version and the exact same thing is happening to me. I already tried installing a Beta version for gboard (7.8) but the issue persists. I noticed it gets corrected if you uninstall the updates for gboard but as soon as you update gboard it defaults the vibration to the system vibration although in the gboard settings it still shows 0.1ms.
I tried uninstalling gboard package from miui completely using the adb shell:
"adb shell pm uninstall --user 0 com.google.android.inputmethod.latin"
&
"adb shell pm uninstall com.google.android.inputmethod.latin"
Then I reinstalled from the play store and the issue persists. I'm trying to find a fix but for now the only work around is to leave the original version installed and don't update...
If you get any luck please let me know! I hate to use the default vibration and I don't want to disable the vibration completely. This is a very annoying issue and I'm 100% sure it's related to an incompatibility between MIUI 10 and newer Gboard versions.