Whille not a bug this is a way to use dungeons and dragons rules to your favor.

I will teach you how to get the Combat Expertize feat defensive bonus while still attacking at max rate.

Its really simple. Combat Expertize feat increases 5 to AC and takes away 5 from attacks.

If your life is low just take your main weapon and start using the shield. Use Combat Expertize and gain its benefit

You will start attacking with the shield but the shield is not penalised. You will only attack once per round as the shield attack is a full round but you will still benefit from full attack bonus, meaning, Combat Expertize feat does not apply to shield attacks.

Now try it.