The reaper item buff does not increase lifesteal

Title, the buff from reaper increases your strength correctly, but the lifesteal is always 25%. Theoretically at 6 stacks and with the 4s buff lifesteal should hit 42.5% but, again, it doesn't apply and you only heal for 25%. If anyone who understands calcs in this game has a reason for such I would like to know. Test yourself, and lmk.