> This may happen once in a while due to some algorithm weirdness, but it is absolutely not "often" the case

It's completely predictable, in fact I checked this claim at Google Flights before posting.

Say you want to fly from LON to NYC, next week, on OneWorld airlines, nonstop. Check the price for a one-way, then check the price when you add a return six months out.

No. It depends on the market, but if it is what's known as a "round-trip" market (typically international itineraries), it shifts to become "often" the case.

Airlines are all about price discrimination. Leisure travellers benefit inordinately from this behavior.

It is approximately always the case for international flights.