Still today, mIRC is being promiscuous and utilizing whatever adapter it feels like.
The latest beta is working correctly for me when I enter a specific network adapter in the ports dialog.
Can you describe a step by step method that reproduces the issue you are seeing?
Also, remember, if the network adapter you have specified in the ports dialog is not available for any reason, mIRC will default to using any adapter for the connection, eg. if the adapter is disabled or no longer exists.