What was your routing? If the segments were over 2,000 miles (e.g. LHR-DOH) then you should still get 35 TPs on a basic economy ticket.
I don't think this has anything to do with changes from 28 April. It sounds like there might have just been an error.
This is incorrect. It has everything to do with the changes from 28 April. Previously you'd earn 35 TPs in lowest economy fares, now you earn 20.
OP, see the thread here:
http://www.flyertalk.com/forum/briti...28-4-15-a.html
It applies to all partners, not just AA. BA will credit you the difference 'as a gesture of goodwill'.