Hi,
I tested LEDE snapshot on my R3G, and it seams that there is an issue with the integrated switch and VLAN support.
After some search, I found the same issue on a post about D-Link DIR-860L (which use MT7621) : https://forum.openwrt.org/t/mt7621-dir-860l-does-not-support-vlans/1714
And the workaround is the same for the Xiaomi : VLANs need to be incremented to work fine (if you want to use VLAN 1 and 3, you need to add VLAN 1, 2 and 3).
I don't see any patch for this issue, do you think that could be corrected in future release ?