There is one known UA issue that may cause this from time to time. UA's system does x searches in total for the roundtrip and then stops. When doing OWs, it will do x searches on the outbound and x on the return, effectively 2x , twice the number of searches for the roundtrip. So the OWs search is deeper and finds more options.
Use Google Flights instead and it will send you to UA to do the purchase
As UA/LH have a TATL JV, UA really does not care which you purchase.
Is the preferred UA routing longer connections or trip time?