Good news! :-) I tested with internal mode on my usual setup and it works fine for me. I did a round robin of 4 to 5 songs going from one machine to the other and only using Ctrl+F7, load next song, Ctrl+F3 and the transfer from broadcast to listen to broadcast worked fine! I did not yet test external mode at all although I have some hardware for doing that in future. So from my perspective it is working as expected now in internal mode.
Having said that, my only MIDI interface in my collection is the one I am using (SIDKick Teensy with MIDI expansion). I have a Messiah but don't see it fitting in at the moment as a MIDI interface. So other MIDI interfaces may behave like the emulation you are focussed on, who knows? Or to turn it around: If SIDLab needs a hardware MIDI cartridge that always provides valid values when read - it would not be too hard to create a new MIDI cartridge that satisfies the special needs of SIDLab I guess.
To prove my writing I will try to attach a few screenshots.
