The key point here is that compact block relay is only as effective as the similarity of transaction sets across participating nodes. When mempools diverge too much the protocol loses its main advantage which is the ability to send small messages and rely on peers to already have most of the transactions. If a node implementation chooses to selectively withhold or inconsistently forward transactions this increases variance between mempools which in turn forces more full transaction data to be sent during block propagation. The result is higher latency and increased bandwidth use and in the competitive environment of Bitcoin mining this translates directly into a higher risk of stale blocks.