Ok, weird. I decided to wait for replies on this thread, and in the meantime disable the packet filter with pfctl -d and work on other bits of the config. I went to gateways and found the IP for the active gateway was no longer showing (this was previously automatically added in and I'm certain it was showing as 192.168.50.1). Added that IP in, rebooted and the pass-all seems to work.
I...still don't know why this would present in logs as it did.
I...still don't know why this would present in logs as it did.