Yes, timer count down won't auto back to 21days after each completed hatch, even it is unplugged for a long time.
We programmed it like this is because it is normal sometimes some chicks will later hatch until 22~23, if timer will auto back to 21 days after countdown to 0 day, then on day 22, the egg turner will rolling again, if at this time someone forget to remove the egg turner, will cause the egg turner turning an unhatched egg on day 22, this will affect normal hatch, so this turner won't rolling when count down to 3 days, and won't auto back to 21 days, unless manually operation.
Suggest to use "reset" function which is easier, to reset timer to 21 days, when every time prepare for a new round of hatch.