Not rendering certain MIDI notes, random

[Flaneurette]

When I have a project with about 24-30 midi tracks, I noticed that Reaper sometimes doesn't seem to render certain MIDI parts. Or I hear a single MIDI note lingering, where there should be a MIDI chord instead of a note. On playback, it sounds fine but not on rendering... it happens randomly and I have to relisten every render to see if all notes where correctly rendered, which is rather time consuming.

CPU is about 30% max and I set it to time critical in Windows to have enough CPU power. This did not make any difference... I use the latest version of Reaper.

Is this a bug? and if so, what could it be?