TR-808 / TR-909 - step mode issues

I have a TR-808 and a TR-909 connected to my Hapax via MIDI, separate outputs on the Hapax and separate MIDI channels. If I play the drums in live mode, I can happily trigger multiple sounds at once. If I enter step mode however, only two sounds seem to be playable at once, with the third step simply ignored / not played.

I can’t find any config in the Hapax that would relate to polyphony and have also ruled out other issues such as:

  • Both drum machines can sound three notes at once using their internal sequencers
  • Likewise with MIDI from Logic
  • No issues in live mode

Does anyone have any ideas or has anyone experienced this?