recvmsg() "short receive" after FIONREAD

I observe a problem with the code that can be seen here:
https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/blob/master/src/modules/rtp/sap.c#L142

The code uses ioctl(FIONREAD) to check the size of available data in a socket.
Does / should this work?

Then the code calls recvmsg() on the socket with single vector with iov_len 
equal to the size obtained earlier.

But the return value from recvmsg() is smaller than the iov_len value.
In my test I see 215 vs expected 263 (so, the difference is 48).

Does this ring a bell to anyone?
I see this on a month old 14.0-CURRENT arm64.

-- 
Andriy Gapon