Has anyone gone off mirtazapine and become an insomniac the very next day?

Yes, I had the same experience. After four months of taking Mirtazapine I tapered from 45mg to 30mg for three days. Then I stopped the Mirtazapine and started 10mg of Trintellix. I didn't sleep the next four nights.
I notified my doctor and we decided to temporarily add back 7.5mg of Mirtazapine at bedtime. That enabled me to sleep well again. Coincidentally I also take Lamotrigine (450mg/day).

Along with other antidepressants, I had been on mirtazapine 30 mg for almost a year, few weeks ago it was reduced to 15 mg, and at least for the first few days, i wasn't able to sleep well, i wasn't knocked out like i was before, and i feel the culprit is mirtazapine, if you are used to it to help you fall asleep, you're bound to be sleepless if you switch. I'm no professional but I've read one of the main symptoms of mirtazapine is drowziness, also heard that you shouldn't take it if you're planning to do anything soon. I've gotten used to the 15 mg now and am able to sleep, that also could be because I've never had insomnia or issues with sleeping. I'll be very scared to completely be off it myself, so no you're not alone, if it's making you really insomniac you gotta bring this up to your psychiatrist if you haven't already, and good luck, the maoi's are intense, hope that's going well.