Had the opposite the other day when booking ROC-ORD-SFO-EWR If I booked it as multi city my SFO-EWR flight was $249 vs if I did 2 OW's my SFO-EWR dropped back to $149.

Took the ow's but the pricing diff might have been caused due to Z fare outbound and Y class return(forgot the fare class K, T, or Q i believe )

Even last year when I was looking for flights for EWR-FRA I wanted to do 2 OW's as I had some etc and was trying to cut costs, RT would have been anywhere between $800-$1000 but each OW was $2600 Just did a dummy search on Google flights and similar result came up today $810 rt or $2800 ow