It certainly is the case with CX. One of the things that led me to try Emirates was attempting to book CX F from LHR to AKL around the time they dropped F to AKL. Fares did not change for itineraries with flights HKG - AKL in J compared with those a week earlier when they offered F. IIRC the website even pointed out that although they were charging for F one sector was in J.
I didn't agree to purchase such a ticket

Now whether the EK F lounge guardians will go on the class shown on the onward BP or booking class I'm not sure. I have suspicions that the OP may be in for a bit of a discussion...