When I have fast render on (from optifine) the blur effect doesn't work. But when I turn fast render off the blur effect works just fine. If possible fix soon! 👍

No, Optifine. and especially fast render, is not and won't be supported.

Optifine's fast render option is there to increase performance by using an alternate rendering method that removes certain fancy rendering... what did you expect? Blur is fancy. It goes bye-bye if you choose to remove fanciness.