In recent MacOS BigSur update; port 55555 is blocked

With the new MacOs Big Sur update, its looking like port 55555 is blocked by the operating system. A query to the list of processes that are using that ports return nothing lsof -nP -i4TCP:55555 | grep LISTEN which is expected. However when attempting to start a server on that port an error is received:

nc -l 55555
nc: Address already in use


The issue here is we have a product with 15K+ docker downloads that depend on this port and up until now it seems a bit mysterious why this port is blocked by the OS only on BigSur; non of its predecessor OSs had this port blocked.

Anyone is facing this issue or has an answer to why is this the case? Thanks!
I’ve seen the same issue on iOS 14, with the same underlying cause. In a recent iOS release we freed up UDP, meaning this only affects TCP. See this post, and the surrounding thread, and the threads it links to, for more background.

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
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In recent MacOS BigSur update; port 55555 is blocked
 
 
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