Ofc they are doing this. Institutional buyers are not one mind. The managed funds which rules allow these trades are likely buying early. That is what they paid

Ofc they are doing this. Institutional buyers are not one mind. The managed funds which rules allow these trades are likely buying early. That is what they paid to do. The Index funds are "stupid" or mechanical. They rebalance on as their rules indicate, but they are cheap. I am not even sure is it illegal to front run your own fund with other fund... Buy before rebalancing and then sell it to indexed fund at market price...