Single commit is lost from the validator nodes

[shuse2]

Expected behavior

When the network is behind on certificating new height, the validator nodes should keep the commits for the height which must be certified. (ie: the one height before the BFT parameter change are made is required)

Actual behavior

Some validator loses the commit created and due to that, the network won't be able to catch up with certification

Steps to reproduce

Observe lisk-core testnet which runs 6.0.0-rc.2 SDK

Which version(s) does this affect? (Environment, OS, etc...)

6.0.0-rc.2