When attacking with a shield on, attacking more than twice will act as if the shield is not equipped.

Before attacking

After attacking

Unequipping it and re-equipping the shield fixes it

Nothing is causing shield attribute modifiers to drop with just Apotheosis. You'll need to play around with your mod instance to figure out what's causing it and report it to them.

I suspect you have another mod causing this issue, apoth does not manage the application of attribute modifiers to the player from equipment.

I'll check it out in isolation though.