We fly one way all the time, because I book with miles and it works out better that way. Literally (and I mean that in the true sense of the word) every single trip since our 2011 trip to Spain. That’s 10 trips to Europe, one to Australia, one to Patagonia, and two to Japan. These are as recent as September to Europe, and right now we are in Japan—-we flew from Seattle to Tokyo 4 days ago. Japan Airlines did not bat an eyelash.
The only 2 times we were asked for proof of a return ticket was when we went to Chile; I anticipated this and had a copy of our return flight to show. Also on an Emirates flight from Sydney to New Zealand. Again I e pelted this and had proof of a ticket back to Australia. But only those two.
Airlines we have flown are British Airways, Condor, Qantas, Emirates, ANA, and Japan Airlines.
Maybe it is a Delta thing?