Attributes with same UUID don't stack when wearing armour

When putting on pieces of armor, the attributes didn't stack up. I had on a pair of boots that gave me -10% speed, a chestplate that gave me -25% speed, and a pair of leggings that gave me -35% speed. However the effect applied to me was only the one of the last piece of armor I applied, not all of them added together. The operation applied to each piece of armor was 1, so in theory I should have had -70% speed, but it was pretty obvious that that was not the effect I had.