When a player holds both an axe and a shield(whichever the dominant hand is holding), it seems that shield is at a higher priority level than axe. So, when facing a waxed copper stuff(block, slab, etc.), shield is considered first, and the player would hold up the shield instead of unwaxing the block.
Duplicate of MC-271403 (Works as intended). You have to crouch to unwax blocks while holding a shield.