We have been having a problem with 10.5.sdk, where midi messages were not being added to the processors midi buffer. Went through and found that the problem was related to a change between 10.4u and 10.5 sdks. The parameter list of startnote in 10.5 is different, the last patameter inStartframe needs to be UInt32 instead of long as in 10.4
You have fixed HandleMidiEvent which had a similar problem between sdk versions. Keep up the good work!