I found a fix in 3.4.1 upstream for retractions on support with a single extruder;
Original ticket: https://github.com/Ultimaker/CuraEngine/pull/709
Two commits involved in the fix:
When merging, please incorporate this fix as well
@victor_larchenko we will be out for the next couple of days, and will not be able to test the recent branches you and tomofey have been working on.
Please start merging in cura engine 3.6 to a separate branch. We may have a couple small fixes upcoming, but this will be the heavy lift.
@TK if all your other tickets are awaiting testing, please lend a hand as well