It’s still environmental at the heart of it. Consider a different scenario involving two laser-trapped atoms isolated from the rest of the universe and initialized in a superposed, entangled way such that Atom 1 is in state A and Atom 2 is in state B or Atom 1 is in state B and Atom 2 is in state A.
Upon measuring Atom 1’s state, there is a 50% chance (say) of measuring A and, immediately after that, Atom 2 would be measured as B with a 100% chance.
But the environment interferes. In practice, Atom 1’s state might not be 50%/50% A/B, and Atom 2’s state won’t be 100% determinable from the measurement outcome for Atom 1. Coherence is lost with time, and it’s hard to fight even in extremely controlled settings.
For the nucleus example: the state “The nucleus has decayed” is much much less isolatable from the environment – so much so that one would never even think about the superposition of the pre- and post-decayed states in any practical situation. After all, stuff comes shooting out into the environment immediately, and there’s also no way to practically set up an experiment that could reveal a superposition without also measuring a specific state for the nucleus in the process.