(Audio) MIDI doesn't start until fullscreen toggle

Description

On Windows 10 using Retroarch/Libretro, I started Touhou 2: Story of Eastern Wonderland, and when switching from FM music to MIDI music, it stays silent, though toggling fullscreen with F gets the MIDI started (with incorrect instruments since the song started from the middle, which is fixed by starting a new song) Also, the problem comes back when you die and are booted to the main menu.

I also tested Doom with DOSbox and have the same problem, so it seems like a RetroArch Bug.

Expected behavior

MIDI just plays when told to

Actual behavior

MIDI doesn't play until pressing F to enable or disable fullscreen, and this workaround only works if MIDI music is currently playing.

Steps to reproduce the bug

On windows 10, use Neko Project II kai core
open TH02.hdi as "content"
You may want to speed up the CPU clock multiplier to 52 or so in core settings
in the game menu, select "Option" then use the right and left arrows to select MIDI
Music is stopped until pressing F (without game focus mode)

Bisect Results

I currently have Sound Canvas VA installed and am using loopMIDI, SAVIhost, and MIDIMapper Configurator based partially on Phil's Computer Lab Sound Canvas instructions https://www.youtube.com/watch?v=WumRboSfn90 (except the MIDImapper part), though I'm not sure this has anything to do with my issue.

I have tried using the Microsoft GS Wavetable, but that has the same problem.

MIDI plays fine in Windows Media Player with Sound Canvas VA as well as Microsoft GS.

Version/Commit

You can find this information under Information/System Information

RetroArch: [version/commit] Build Date: May 10 2019 Git Version: 52dd39f98a

Environment information

OS: [The operating system you're running] Windows 10 x64 Build 18362 (v10.0)
Compiler: [In case you are running local builds] downloaded from retroarch website

original bug: https://github.com/AZO234/NP2kai/issues/73

libretro / RetroArch