The Bug:

Placing a block with your offhand resets your mainhand weapon cooldown

How to Reproduce:

Place a block with your offhand and then quickly attack an entity with a sword

Observed Behavior:

The weapon does a weak attack, not producing a sweep hit and doing less damage

Expected Behavior:

Placing blocks in offhand would not affect the cooldown of your mainhand weapon

Since this bug doesn't happen when using your offhand to eat food or shoot a bow, I'm assuming it has to do with the swinging animation that happens when you place a block (the same animation that plays when you attack).

In the attached video you can see the fully charged sword attacks producing a sweeping effect, but after placing a block the sword isn't charged, so it does a weak attack instead.

weapon cooldown bug.mp4