Prevent date/time format from changing when updating hosted feature layer AGOL

[cmbappleby]

I'm updating a hosted feature layer using a Python notebook that basically does the following:

1. Reads new data into a pandas dataframe
2. Adds three new columns on the end and calculates values
3. Adds the new features to the feature layer

The first column's values are in ISO 8601 date/time format, but when I add the new features, it changes to a generic date/time format. The field data type is string, and the dataframe data type is object. Is there a way to prevent the ISO 8601 date/time from changing to a generic date/time when I add the new features?