The problem/bug is described in the Plotly-Dash animations documentation here:
Animations are designed to work well when each row of input is present across all animation frames, and when categorical values mapped to symbol, color and facet are constant across frames. Animations may be misleading or inconsistent if these constraints are not met.
My example has daily frames, but each day does not have the same number of hours of data. Jan 19 has two hours, while Jan 20 only has one hour.
To fix the problem, I need to make each day have the same number of hourly “charts” (e.g. 24 charts of data each day).