Astx share price is amostly exactly where it was at the last rebalancing in March. The methodoly change reduced the cap on companies from 8% to 5%. The overages are rolled down and then stocks that are raised to over 4% are capped. Since Astex is small relative to the large market caps in the index that should have increased Astex's weighing not decreased it.
Do you have the weights before and after June 21st?