I am on 6.40.4 (same issue with 6.40.3)

I have 2 EoIP tunnels between two locations. One for a LAN subnet, and another for a WAN subnet.

I am having issues with the WAN EoIP Tunnel / bridge setup.

On router #1 the router has an IP on the WAN subnet. One router #2 the router does not have an IP on the WAN subnet. When configured this way the tunnel works correctly and there are no issues and this feature is really useful. For both sides they bridge with an ethernet port which plugs into a switch that connects a small group of machines (about 6)

However, when I add an IP on the WAN subnet to Router #2 I always get a "transmit loop detected, downing interface for 60 seconds". I have disabled loop detection on both the EoIP interface and the bridge (on the bridge setting STP to none) and I have done this on both sides. It doesn't seem to make a difference and I still get the loop detection error. The loop detection error occurs on Router #2. If I disabled the IP Address on Router #2 then the error stops.

Why would adding an IP address to the bridge cause any kind of loop? If this only happens when adding an IP is this a bug with MIkrotik, or is it a problem with my topology or setup?

I was struggling with this as well. I figured out that it was a routes issue. I had bonded some EOIP interfaces together, and the packets would travel over the bonded interface as soon as you added 0.0.0.0/0 to routes.

So I had to do a firewall block on the bonded interface to stop any of my eoip packets from traveling over the bonded interface.

I added
/ip firewall filter
add action=drop chain=output dst-address=10.40.0.5 out-interface=MerleBridge
add action=drop chain=output dst-address=10.40.0.1 out-interface=MerleBridge
add action=drop chain=output dst-address=10.40.0.9 out-interface=MerleBridge

Where 10.40.0.x were my remote address' for the EOIP tunnels.

Hope that helps.