Adding 25 grains to the tip will make WAY more difference to the effective spine of an arrow than cutting off 1/2 inch.

It's really hard to believe that 1/2 inch made such a huge difference. Are you sure there were no other changes (lighter inserts, less epoxy used for the inserts, inserts not in straight on an arrow or 3, arrow rest moving without you realizing it, nock set sliding up or down the string, etc)?