Generally you should try to keep the update period in OpenMPT's sound settings as low as possibile for jitter-free MIDI output - 5ms or lower. OpenMPT's MIDI output is unfortunately not jitter-free in its current design, as audio is always processed in chunks (defined by the latency and update period) and if one such chunk contains more than one tracker tick (which is quite likely to be the case), the MIDI output of those two ticks might be grouped together and be sent at the same time.
I haven't tried it, but maybe Tobybear's
MIDIBag would work better for you, as it would avoid having to go through the real-time MIDI loopback ports: In OpenMPT, you would create VST2NET instances (sending data to
localhost), and in Reaper you would add NET2VST instances to reverse the process.