Maybe you just didn't notice, you might have them since the beginning. Some percentage of bad blocks straight from the factory is tolerated.

Bad blocks don't hurt unless they are in the first block or two where the lowlevel bootloader is. config/kernel/initfs/rootfs partitions can cope with it. At worst you loose some data or rootfs will be unreadable and will need reflash. At least kernel and initfs partitions can overflow so in case of bad block next blocks are used and stolen from next partition and the beginning of next partition(s) shifts. That's also why I'm guessing your bad blocks come from factory or otherwise you would notice that rootfs shrinks and becomes unreadable. It happened to me.