I've created multiple dimensions using both cheated in time noon dimlets and a legitimately created one (holy cow was that a pain!). I don't know if they all had a normal day/night cycle, as I didn't always stay long, but at least some seemed to completely ignore the Time Noon dimlet. Shift-right clicking the tab gives the following:

@tflat,Bminecraft:smaller_extreme_hills,Csingle,eNone,fNone,SNone,tNoon,d0
Digits: 0
Base seed: -3889462089294469064
World version: 1
Terrain: TERRAIN_FLAT
Base block: Stone
Base fluid: null
Biome controller: CONTROLLER_SINGLE
Biome: Extreme Hills Edge
Feature: FEATURE_NONE
Structure: STRUCTURE_NONE
Sun brightness: 1.0
Star brightness: 1.0
Sky color: 1.0, 1.0, 1.0
Fog color: 1.0, 1.0, 1.0
Celestial angle: 0.0

Did I mess this up or is this a bug?

Looks like rebooting the server fixed it. The dimension is now eternally noon, as it's supposed to be. Must be weirdness with Minecraft and dimensions.