In my place I have both R15 and R6 humanoids in the same game and :TakeDamage() starts acting funky. TakeDamage() will always work on the R15 humanoid of other players, but it’ll only work once on the R6 humanoid of Bob, all 3 bobs can be killed but once they respawn they cannot be killed again.
However if I turn off R15, Humanoid:TakeDamage() works as its supposed to.
Repro: Make the player into an R15, make an NPC into a R6, kill NPC, let it respawn and try to kill it again.
EDIT: I stand corrected, TakeDamage will almost never work on an R6 humanoid if R15 is enabled.