Some mobs (eg. Skeleton) spawn there frozen.
When you attempt to break block under them.
They kind of floating.
When you attempt to hit/kill them its only one hit.
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Confirmed. Happens when near a ton of Redstone
When I tested an "op Zombie" command, the zombies freeze for a while and so do any other entities (like pigs, sheep and the like...)

That's lag due to the amount of redstone
This is a duplicate, but I don't have the time to searh for the ticket

@unknown that is due to the redstone, lots of redstone lower the preformance of minecraft, as it has to do a lot more in the same amount of time.
To fix your issue lower the amount of redstone, or allow minecraft to use more memory