mutations that occur after reproduction are never weeded out since the genes are passed on already, so natural selection on its own can result in this, just like cancer or lower safeguards against cancer such as short telomeres in humans

This isn’t quite true, is it? A mutation that killed 5 years after childbirth would result in higher mortality rates for the child.

Octopus have extremely high mortality rates and extremely large litters, so there’s that

They don’t raise their offspring

Though FWIW, they do protect their eggs quite well.

Isn't this tied to the senesce? Where their digestive tract has closed and they starve to death at the nest

Ah yeah, I was thinking humans