Thanks for the code.
I think something has changed. There seems to be no support for the pro micro any more. There is the "Arduino Micro (MIDI)", but it seems not to be compatible with a lot of things that used to work like the EEPROM, SPI and so on. The messages I get about these are:-
WARNING: Category '' in library EEPROM is not valid. Setting to 'Uncategorized'
WARNING: Category '' in library SPI is not valid. Setting to 'Uncategorized'
WARNING: Category '' in library SoftwareSerial is not valid. Setting to 'Uncategorized'
WARNING: Category '' in library Wire is not valid. Setting to 'Uncategorized'
Warning: platform.txt from core 'Leonardo & Micro MIDI-USB (arcore)' contains deprecated recipe.ar.pattern="{compiler.path}{compiler.ar.cmd}" {compiler.ar.flags} {compiler.ar.extra_flags} "{build.path}/{archive_file}" "{object_file}", automatically converted to recipe.ar.pattern="{compiler.path}{compiler.ar.cmd}" {compiler.ar.flags} {compiler.ar.extra_flags} "{archive_file_path}" "{object_file}". Consider upgrading this core.
My own stuff that was working and compiling about eight months ago will now no longer compile as well. I have dropped back to the previous two IDE releases and still no joy.
So unfortunately at the moment I have no solution, but as a lot of my stuff is also suffering I will be working on it and get back to you as soon as I have sorted it.