Have had a "chat" with Samsung and been advised I will receive an email re my technical problem. My Galaxy Tab A 10.5 will not receive MIDI from my digital keyboard. USB debugging option in Developer options is selected. I am trying to use the app "Simply Piano" The MIDI output from my keyboard is working correctly on my laptop.
Had the same issue ... fixed it by
switch to developer mode (search youtube how to do it)
settings - developer options - disable USB audio routing
Setting - Note recognition - Use Android MIDI checkbox
P.S. BAD cable
Here is another good link
I have the exact same problem, but I'm not sure if Samsung is to blame. My keyboard is a Roland Juno-DS76 and I have it connected to the tablet using a USB OTG adapter. If I use the (default) vendor driver in Simply Piano, MIDI as such works - but the stave animation hangs every once in a while when I'm playing, which is extremely irritating. When I follow the instructions above, i.e. use the Android MIDI service, disable USB audio routing etc., Simply Piano won't detect the connected keyboard at all, just as the OP describes.
I tried quite some different Android devices from HTC, Huawei and Samsung and all of them showed the same behaviour. I had lengthy interactions with the support staff at Joytunes, but they are as puzzled by this as I am.
I have ordered a different OTG adapter as well as a MIDI cable that connects to the keyboard's MIDI ports instead of the USB port to check if that might help. I haven't got the hardware yet - wish me luck for when it's here. In the mean time I'd massively appreciate any further ideas for solving this problem...