"Limit IP Address Tracking" versus "Mail Privacy Protection"

Note: 

If you deselect the checkbox in Network preferences to limit IP address tracking for your Wi-Fi or Ethernet network, your IP address isn’t hidden from senders when using the network.

Those two things are not the same. It isn't that it is hiding the IP address as it is loading the remote content through two independent relays. That does hide your IP address, but the point of the two relays is to prevent the spammers from knowing that their payload was delivered to that email address.

On one of my two Macs, Mail loads content privately. On the other, I get the irritating message, “Your network preferences prevent content from loading privately.”

The one that works is a Mac mini. 

• System Preferences > Apple ID > Private Relay (Beta) is unchecked. 
• System Preferences > Network > Use iCloud Private Relay is absent. 
• System Preferences > Network > Limit IP Address Tracking is checked.
• Mail > Preferences > Privacy > Protect Mail Privacy is checked.

Mail loads email content automatically.

The other computer is a MacBook Pro. 

• System Preferences > Apple ID > Private Relay (Beta) is checked.
• System Preferences > Network > Limit IP Address Tracking is absent.
• System Preferences > Network > Use iCloud Private Relay is checked. 
• Mail > Preferences > Privacy > Protect Mail Privacy is checked.

Mail fails to load content automatically. 

When I uncheck the MBP’s System Preferences > Apple ID > Private Relay (Beta), the System Preferences > Network > Use iCloud Private Relay disappears, as it should. But it is not replaced by Limit IP Address Tracking as it should be, so it can’t be checked, and the annoying message persists.

Apple’s support document About iCloud Private Relay explains it entirely in terms of Safari, never mentioning Mail.

The "learn more" button in Mail Preferences indicates that Mail Privacy Protection routes the remote content request through two different relays to obfuscate your address. I don't think it is related at all to Limit IP address tracking.

I have Mail Privacy Protection and Limit IP address Tracking enabled without any issues. All remote content in my email is loaded.

What I still don't understand is that when Limiting is checked on, Mail Privacy Protection cannot load remote content privately (I get the "Your network preferences prevent content from loading privately."); however, if I uncheck "Limit IP Address Tracking", the Mail app does not complain and loads remote content (though I have no way of knowing whether the remote loads are done privately or directly).

I guess my question boils down to: why does turning off LIAT fix loading remote content privately in MPP?

I don't know, and it works correctly when I have both enabled.

I guess there is a clue in there, somewhere, but I don't know what it is.

On another thread, the OP disabled MIMO and UPnP. Both of those restarted the router. The OP re-enabled them one at a time, and everything worked correctly even with them enabled. I think restarting the router may have been the fix.