I'm looking to book a trans-Atlantic flight. My options are basically United and Lufthansa; each operates some flights, and each sells tickets on the other's flights. For one leg operated by United, Lufthansa has a premium economy ticket available on their site, but only if I book as a round-trip (if I try multi-city it says premium economy is sold out). United, on the other hand, says the premium economy is sold out on their site, and as the operator I assume they'd have the most up-to-date info. Is this just a website glitch, or is it possible that the premium fare is in fact still available?
Apparently Lufthansa is a little confused in some way or another.
United Airlines has only recently started rolling out Premium Economy to it's aircraft, and will only launch this class on March 30, 2019, and even then only on a few select routes. Whilst it IS possible to purchase tickets in Premium Economy (and has been since early December), they are only for sale on flights on/after March 30.
As your flight is before March 30, United is not selling ANY tickets for Premium Economy, and nor are such sales available to their partner airlines, including Lufthansa.
Always, always trust the operating carrier over a codeshare partner in this regard. They know best what inventory they have.