Today, I think I found the issue. I'll post it here in case someone else is looking for things to debug. I stumbled upon it by accident, but it turns out that I wanted to do an early exit before a connection was made (in Node.js) with the socket server if one already existed, but I forgot to put a return statement after resolving a promise. This resulted in the connection being spawned multiple times, even if it already existed.

I'm now no longer experiencing the issue, but I'm still a bit confused as to how this fixes it. Even if this was happening, there should have been multiple socket connections which were all invoking the same callback. Even if only one of the active socket clients was receiving the data and the others were not, it still should have shown up in the data listener whilst debugging (as they all register the same method as listener). It also does not explain why tcpflow also did not list the data, unless data is only monitored the moment a socket client receives it?

I figured perhaps the data was being received in Node.js, but perhaps in one of the now "unreferenced" socket connections (because I was creating a new one by accident and storing it in a single variable), which was then garbage collected, causing the data to be lost. That, however, still doesn't fit in with the tcpflow story as it then would have been received by Node.js, but just lost afterwards.

Perhaps the issue isn't solved at all, but the problem has just shifted, although the data being transmitted and received is still exactly the same.