I am playing a flat world in survival mode, growing many oak trees from a sapling in the bonus chest. There are no flowers around, only grass. After at least 30 or so oaks were grown, I heard the noise of a bee, looked around, and discovered a bee nest at ground level in one of the oaks, with a bee searching in vain for nonexistent flowers.

I thought there was a rule that for a 5% chance of a bee nest spawning in an oak grown from a sapling, there must be a flower within a couple of blocks of the sapling.

This wasn't a glitch either. It happened again, and now there are two bee nests near my home base.