I finally tracked down the problem. A departed member of our team incorrectly changed the lessEntryPoints in Build.scala a week ago so that there was no inputs being passed to the less compiler. I guess that Play does not delete CSS files which are removed, so it took a week for the problem to show up until any of us ran "play clean-all". It's also very strange because I synced my project's git repo back a couple weeks before he made that change to check if it was on our end and I still had the problem, so maybe there's some caching of the sbt config happening or something. I'll try to investigate later to see if I can come up with anything reproducible to demonstrate unexpected caching.
Thank you guys so much for testing the assets compiler and letting me know it works for you. It was very helpful.