This happened to me yesterday (Tuesday). Before I saw the email, I logged onto
MDE and saw that my FP+ for Illuminations last night was just GONE!
I called Disney, and the rep said that although my FP+ was gone for unknown reasons, I had been granted in its stead a super-duper FP+ that was good at ANY attraction at ANY park ... including Illuminations!

Or, if I wanted, Mine Train, Soarin', TSMM ... literally any FP+ attraction.
But I really wanted to see the Holiday Finale on Illuminations, so I went to Epcot & went to the Preferred Viewing Area. Illuminations went on as scheduled, and I watched from the PVA along with about
2 dozen other Guests. That's right ... the PVA was virtually empty! And I had checked MDE earlier and it said all the FP+s for Illuminations had been distributed for the day.
I suspect something glitched in the FP+ software yesterday morning.