Hi @AlixVezina, I added Previn. I tried creating the date field in both AGO on the feature class directly, as well as in Pro, but same thing.
The most recent date field added in May actually did have this same issue at first, but eventually resolved itself while I was troubleshooting (unfortunately I'm not sure what fixed it at the time). At the time I thought it was a fluke and didn't think much of it. I went back and found a screenshot from the primary user of the app:
However just now I created a fresh feature class within AGO and added a date field using AGO + Pro, and both are working fine. Removed time reference and changed alias, still fine.
Next I exported a local copy of the medians dataset in Pro, and republished it as a fresh web layer. No issues. So... I guess it has something to do with my layer was published originally, or something with my specific layer.
I should have tried these troubleshooting steps sooner, I apologize, but thank you for looking into it with me. Still quite bizarre though!
Have a great day and let me know if you'd like to try anything else.