Weirdest thing is that it sometimes works the other way as well. I had a Flex J ticket and I had to do a date change online .One of the available options was a F ticket (for a £30 change fee).

I assumed that I was just lucky and this was some weird saver F fare which cost the same as a Flex J. However MMB showed flex miles for F and after the flight I was credited with Flex miles for F as well. I still havent figured this one out.

PS: I did a lot of dummy bookings around the same dates and flights to see if there were any special sale fares but did not see any..!