Amaranth is prone to fungal diseases in very wet weather and that's what probably happened to yours. You have to be very vigilant to catch it before it spreads

Amaranth is not the only plant in Canada to succumb to fungal disease this year. People who have never had them before are getting them this year, so don't feel that you killed them.

If you plant them again make sure you have well drained soil and water deeply when you do water and them leave them to dry out before you water again and keep a close eye out for discolored leaves and drooping. then treat with a fungicide