MTProxy when connected to VPN

[Forever-Young]

I have simple setup: only one server, VPN, panel, mtproxy - all there.
Users can connect to MTProxy (telemt) and use it - all good.
Users can connect to VPN (certain protocols) and use it - all good.
But when they are connected to VPN, MTProxy stops working, it doesn't connect.
Is there some problem with routing or something, or this is some misconfiguration on my side?
(I can connect to the panel using the same domain and port 443, as for MTProxy, under VPN).
I didn't do much changes other than disabling protocols that don't work for users.
So in short - VPN on - panel works, mtproxy not. VPN off - panel works, mtproxy works.