tinc - Jun 2017 - Why sometimes the MTU discovery by UDP never get a response?

Normally, if the tinc server’s MTU discovery response can reach the tinc client
side, they always can negotiate with a certain MTU size, but the case is,
sometimes, the client sent variety size of MTU probe packet to server, but the
server never respond.

Sometimes, if I change the client side’s tinc port,  the server’s MTU response
can received by client.

BTW, the above case are all regarding the tinc client is reside in a network
behind NAT(not public routable IP)

Any thoughts regarding this? What probably the root cause for this? And what we
can do in order to make MTU probe response always can be received by client?