I have the same / similiar issue.
2 routers.
TCP test works fine as both, receive, or send.
UDP test works fine for send, for receive, it says running, but i get 0 bytes.
If i do both, it sends and receives, and i can see the traffic,but the test results is not the same as when i do send or receive only (probably due to CPU contraint)
One word, firewall (if you have it). Incoming udp test is blocked, because it’s just some udp packets to random port that don’t belong to any established connection. Well, maybe not random (check with Tools->Torch on interface), but in any case unexpected.