Reetala it works either way to the advantage of the current shareholders.
Astellas needs +50% of approx 42 mil shs. or approx 21mil tendered shares
IF there is a cut off date of being a shareholder perhaps you can find that link. There is such a criteria when there is a Proxy Vote. However tendering of one's shares should not matter as to how long one has held their shares.
In any event the new purchasers will not being tendering their shares at $8.50 offer. Therefore Astellas would not get the needed shares of +21 mil.