When we are building blocks for a parachain, we also need to collect a state proof. This means that we will trigger a different code path that currently doesn't support the state cache when we build a block for a parachain.

This code path is taken when building a parachain block:
https://github.com/paritytech/substrate/blob/master/client/service/src/client/call_executor.rs#L234-L260

And this particular call: https://github.com/paritytech/substrate/blob/master/client/service/src/client/call_executor.rs#L234 (as_trie_backend) prevents that we use the state cache.