You can’t have a ‘left channel’ if you are only tapping one side of a stereo feed - you can only tap a mono signal and presuming you don’t use panning on the track then it’ll be fine - no need to sum/merge etc - simple TRS to dual TS splitters are easy to come by or make - just drop the ring between a TRS to TS and it’s all fixed - expecting Elektron to work on this is unrealistic - it’s also very clear the outs are Stereo (as they were on the AK and discussed on here re A4 mk1 DIY track out hacks)
This really is a lot of fuss about a very simple matter to fix with very basic cable juggling or trivial adaptors - the ones Elektron make are good but you can get them way cheaper
Even this type may suffice