I read about this in one of the books- maybe "health sleep habits"? I remember it said something a bad night 4-6 nights in, almost like a final attempt to not be sleep trained. Just keep following whatever plan you started with and hopefully it only lasts one more night
My kid cried on and off at bedtime for months. It was so hard, but it was worth it in the end. We could have 2 weeks of just rolling over and going to sleep and then a week of crying for 10-15mins at bedtime.