Got a call from the complex this morning, just as I was about to call them.
Profuse apologies on their part. My check was received, and I do not owe for any past water bills. What happened was my check got put with the letter I'd enclosed, instead of with the other checks. So it had been remembered yesterday right after the notices had been posted, and they couldn't get my notice back.
This will not go on my record. I am instructed to shred the notice to vacate. I will not be kicked out.
All is much better in my world.