Fix polkadot parachains not producing blocks until next session (#4269)

... a few sessions too late :(, this already happened on polkadot, so as
of now there are no known relay-chains without async backing enabled in
runtime, but let's fix it in case someone else wants to repeat our
steps.

Fixes: https://github.com/paritytech/polkadot-sdk/issues/4226

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Signed-off-by: Alexandru Gheorghe <[email protected]>