RNBO external in Live: MIDI passthrough does not work?

I've been hoping to be able to use RNBO externals to minimize load time of some MPE processing Max patches.

When I write and test the .rnbopat files in Max they work exquisitely.
When I test the .rnbopat files in Live they work exquisitely.
When I export the external and test it in Max it works exquisitely.
>>>> When I test the external back in Live it fails.

Basically, even with a "midi" processor that does nothing, the externals have a low upper limit on how much midi they can deal with before corrupting or not outputting the output at all (I'm not sure which). In practice of more complex MPE stuff, this leads to dropped note-offs, which is disastrous.

What I'd like to know/understand is:

a) Is this expected? Is MIDI throughput like this just not expected to work? It seems okay for pretty minimal loads of MIDI, but its very easy to stress to the point of failure.

b) If (a) is Yes, then why does it work fine in Max?

c) Why the discrepancy between .rnbopat and the external?

d) If (a) is Yes, is this only a problem with passing MIDI through, or do audio externals also suffer from an inability to handle lots of MIDI?