@Phaedrux I plan to end up there, but right now I have fans with mixed voltages. The problem persists when I completely disconnect the 12v fan, so whatever changed seems to have stuck.
My best lead right now seems to be that the DIAG LED stays on, which supposedly indicates that the firmware was erased somehow. I'm hesitant to start the process of reinstalling firmware, but that might be my only option.

Fuses are good and I don't see damage. Buck converter is simply wired inline between the Duet and the Fan. When the Duet is turned on, the fan operates correctly.
It looks like the Firmware was erased. I don't know how I would have done that, but I'm Showing Bossa port instead of Duet 3D Printer Electronics in Device Manager. I have to run out for a while but I'll try to reinstall the firmware based on the link you shared when I get back.

I have a relatively new Duet 2 Wifi. Upgraded an Ender 3 and have made a few prints with it. Today I installed a new hotend fan. Since I am using a 24V PS, I used a buck converter to supply the new fan with 12V. When I turn the board on now, I see the BL touch do its start up and the fan I just installed in P1 turns on and stays on. I can no longer connect via wifi (nor do I see it in a list of available wifi). Also, I can't make a useful connection via YAT. It says there is a connection, but I get no communication from the board and it does not respond to commands.
I disconnected the fan and tried to start it up again and have the same problem.
I did not make any other changes than what I mentioned. Everything was working right until I installed the fan in P1.