I also had the issue with fifth gear skipping only when pedal under pressure.  I tried the derailleur adjustment and it didn?t work.  I then bought the tool to adjust the derailleur hanger and it made some improvement, but fifth gear still skipped under pressure. Took to bike shop, they installed a new freewheel and it still made noise in fifth gear, but didn?t skip.  They installed a 14-28 tooth and I hated it. 6 and 7 gear was not low enough now for the speed of the e-bike.  I decided to put the original freewheel back on myself and as I was removing the rear tire I noticed that the tear drop shaped washer which has the Allen Bolt in it just behind the axel bolt (brake rotor side) was not seated properly and was slightly misaligning the axel. So I loosened up the Allen bolt to ensure it was seated flat, tightened up the axel bolt and then tightened the Allen bolt the rest of the way.  Adjusted the derailleur one more time because I swapped the original freewheel back on and it was FIXED!!  I think the misalignment of the axel resulted in fifth gear skipping.  Was weird that I could get every gear to run fine except for fifth gear, but now it?s fixed.