Odd inconsistency between executing and sourcing Bash script

I know this is not a very descriptive title (suggestions are welcome), but the fact is that I've been pulling my hair over this for hours and I have no clue where the root of the problem might lie. I wrote a simple Bash script for CLI chat between peers on a local network:

#!/usr/bin/env bash

# Usage: ./lanchat : :

# set -x
set -o errexit -o nounset -o pipefail

IFS=':' read -a socket  "$RECV_FIFO"; done &

TMUX_TOP="while true; do cat '$RECV_FIFO'; done"
TMUX_BOTTOM="while IFS= read -r line; do nc -n -q 0 '$REMOTE_IP' '$REMOTE_PORT'  .tmp.lanchat; done &
$ tmux new "while true; do cat .tmp.lanchat; done" \; split -v "while IFS= read -r line; do nc -n -q 0 172.16.0.100 1234  "$RECV_FIFO";
done &
#...

I tried establishing the connection in all possible orders between the two peers. I even restarted both the server and my local machine to make sure that there were no orphaned or zombie instances of *nc* hugging the socket that had somehow evaded detection. Now, Debian and Arch run different versions of *nc*. So, on the face of it, it sounds like this could be a possible explanation. But doesn't the fact that sourcing the script on Debian's side works fine rule out that possibility? What the heck is going on, here?