I created the scenario on my side. Two columns Checkin and CheckOut.
First i saved form with only check in value. And then edit the record and updated the checkout value. It is working fine.
Check your sharepoint field if field is set for taking values in time also. Also are you using SubmitForm or Patch?
Actually default values and all that comes into action when you have unlocked the DataCard and change something. For a normal form with date values t shouldn't be a problem. As i shown you in my pictures. It is working fine. Try this:
Remove Check in field from form. Refresh the database. Add Check in field again. PowerApps sometimes behaves this way
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