Per
xONEx rules
NOTE: For flights where First or Business Class is not offered or available, passengers may travel in a lower Class, in
the applicable booking code for that lower Class.
Where the applicable booking class for the lower class is not available,
Passengers travelling on First Class Fares may book Y class
Passengers travelling on DONE* Business Class Fares may book B class
If premium economy is not listed as a lower class then it will book into economy.
For JL premium economy booking class E is listed, so if available it will book into that.