Just noticed this.
Both these fields, TWebDBCombo and TWebDBDateTimePicker, do not update the dataset. I think they used to work, although I know the date picker didn't update until posting, which was a bit of a nuisance.
The text property has the correct value so can work around the issue in a before post event to directly update the field.
TWebDBComboLookup seems fine.
I'm using version 1.9.6