Just an update on this, it looks like it was something trivial after all 😕
The reason it wasn't working was because only a single time value had been selected. For some reason when I made my selection (today and yesterday's date) it didn't actually save both selections. I needed to deselect today's date first for some reason before being able to select both dates and have them appear as being highlighted. We did reload some data today which may explain the odd behavior though.