Restriction enzymes are usually added after everything else rather than as a master mix. If the added template makes up a substantial portion of the reaction volume (say 25 µL of mastermix added to 25 µL template), then the high salt concentration in the master mix is going to inactivate more salt sensitive enzymes. If you're adding the enzyme before the buffer is well mixed into the reaction, then contact with unbuffered water will denature a lot of enzymes also.

Alexandra Johnson , I see. Thanks for the clarification. so do you recommend I just pre-mix water and buffer, and aliquote into each sample tube and then only add the enzyme as the last step?

Yes, you can do it that way as long as you're putting the same volume of template into each sample. If the added template DNA volume is variable between samples due to concentration differences, then you need to make each reaction individually, because the volume of water added will change between each tube. But in each case, add the enzyme last.