there doesn't have to be redstone on the side of the repeater, if you power the repeater for just a split second, it keeps flashing. this also happens often even if you aren't attempting to do this repeaters keep flashing in all my builds. even without the block and redstone.
This is not a bug. if you are near an elder Guardian, he flashes down your screen, giving you mining fatigue.
I Hope You understand. 😃