Why have my LED bulbs started flashing when I turn them off?
There's likely nothing wrong with anything, it's just that the LED's are much more sensitive to electrical input. Common if there's a dimmer, ceiling fan or 2 (or more) switches controlling the lights.

You need a Snubber or Capload
or RC Suppressor installed in the circuit. Something like pictured in just one place, in a light's fixture or at a or the switch. But, it's fine to just live with it as nothing's in danger, the lights are just waving bye-bye until next time.