Just a heads up for Win11 users using loopback adapters (and very old MIDI devices), you may not be able to see those devices under the new MIDI subsystem that launched with Patch: KB5077181 (Wind0 and 26100.7840 for versions 25H2 and 24H2).
https://windowsforum.com/threads/window ... ul.400821/
https://devblogs.microsoft.com/windows- ... rkarounds/
If you dig through the above two links, you'll find the fix is 2 parts: ensure the driver(s) are listed underneath the 10 device limit, and then restart midi the new midi service to allow the 'old' drivers to load under the older codepath (which is why the 10 device limit is a necessary check in this case). To do this, close all midi client applications (audio and graphics like MMV etc) and start a elevated command prompt (run as admin), and then enter the following two lines:
net stop midisrv
net start midisrv
I use RTPMidi from Tobias Erichsen to communicate with RTP Midi on Macs. While technically not just a loopback, since it's a software midi driver it was also affected. Posting this here in case it helps others.