Replying to this old post just to document what I found somewhere, since this thread came up while searching before I self-solved.
I was able to correct the same situation (on a Ryzen 7 7700x board), by going to Device Manager, then in the View menu selecting Show Hidden Devices, and uninstalling all, even the hidden, instances of the Audigy 5/Rx card and drivers. When you change motherboards or sometimes even PCI slots, Windows leaves the old device tracked but hidden, and reinstalls a new copy. Perhaps something SoundFont-related couldn't be installed this second time even though the original was missing.
Note that I have Memory Integrity protection enabled (Windows Settings App, Device Security, Core Isolation) which blocks the running of the 2018-dated drivers that Windows natively finds, which might have actually helped, I'm not sure. But after uninstalling the hidden one attached to my prior motherboard, then rebooting and installing the latest public drivers, The MIDI device and Sound Fonts were back.