Yeah, it sucks... according to RAW, there's that old (and tiresome
) golden rule which states that bonus with the same types do not stack (exceptions to these are, for example, dodge bonuses)... unless the feat/ability/whatever explicity states that.
In this case, i also agree that they're supposed to stack, because the grapple bonuses are given by different mutations, but i could probably reason that they don't, because "Flexible limbs" are sort of like tentacles themselves, so if you have even more tentacles, it doesn't matter