Auto-Complete as a control does not validate the value selected or typed in manually is valid according to the source list of values.
so as a scenario, I have a region dropdown coming from Data page which has following values:
with this configured in Auto-Complete, I can type in "SG" and submit the form and validation would not trigger for the Auto-Complete value. I have to explicitly do validation in post activity for every Auto-Complete I have on my screen. For this there is a solution proposed in this link.