Well, you’re not entirely alone, there is a general problem with DIN (5 pin) midi on Pi5, caused by issues with the non standard baud rate.
I think you can fix it by updating the bootloader - to do that use RPi imager, for Operating System choose, Misc utility images, then pick Bootloader (Pi5 family).
Then create and run, it should make the screen green to indicate it successfuly rewrote the firmware. This just might fix your issue.
May the force be with you.
Also, perhaps look at this if that doesn’t do it: