After switching item in the hotbar, mining progress doesn't reset as it should, and when the block is broken, the original item loses no durability. Because of this, it is theoretically possible to use a single tool infinitely without losing durability.
A video showing the bug in action: https://streamable.com/x85p6a
UPDATE: It seems that this bug only occurs when the number keys are used to switch item. Using the scroll wheel or the shoulder buttons of a controller reset the durability
It's not a bug