As I did not see any activity on this report, I went out and tracked it down myself (by bisecting the homebrew repository). The problem appeared in version 4.260, 4.258 is fine.
@Andreas_Hegenberg, would you mind confirming and taking a look? This is quite disruptive.
I purchased Better Touch Tool specifically for this purpose, and no combination of settings or scripting I've tried (blocking or async) has managed to share MIDI values with an AppleScript. I even tried downgrading to 4.258 and it still doesn't work.