Everything was done via ArcGIS Pro. The FGDB was built, populated (using local time), and published from there. I believe it was actually the blog you linked to that clued me in to setting the time zone at the time of publishing (I hadn't on my first attempt, and noticed the time/date issues the blog discusses soon after).
The offline FGDB was never time-enabled, but as I understand it, it's still treated as residing in the time zone of my local machine (it's not on a server anywhere). For a few weeks, my workflow of adding data to the FGDB and then appending to the published feature service worked just fine. Date fields are populated without specified times--as I'm entering data on a new feature each date field shows a time of 12:00 AM until I press Enter or save my edits, at which point it shows only the date I entered. When appended to the hosted feature service, the dates all showed up as the date plus several hours, resulting in a display of 12:00am when viewing the data in a web map from this time zone. A couple weeks ago, that just stopped, and now any date that's appended retains the same time stamp without calculation, resulting in an incorrect time date/time displayed in the web map.
I'm sure I've got something completely backwards here, but I'm still not sure why the workflow worked fine for a while and then stopped.
Because the web map needs to run several Arcade calculations based on the various dates, I can't simply switch to using text fields for these, can I? Otherwise, I really only need the dates for this data, and not the times.