NFS client source port on reconnect

I am using NFS client to connect to NFS cluster. I have noticed that the default behaviour when server is unavailable is to retry the TCP connections from the same source TCP port, which I have confirmed by tcpdump (many SYN packets, different seq numbers, but same source port). By default NFS uses priviledged ports (1024 and and now on each reconnect attempt will be from different TCP port. NFS client is SLES12 SP4, the same behaviour is also on Oracle Linux 7.7. NFS server is HAE cluster based on SLES12 SP4. Is this behaviour documented somewhere? Why it does use the same port every time by default but not when using noresvport?