Thank you for your post.
An auto-enter value in a portal will only display after the record has been committed. It sounds like you created a record in EVENTS, but before committing the record, you entered information into the portal. Since the auto-enter cannot find an uncommitted record, it will remain blank. Therefore, you should commit the current record in EVENTS prior to entering the portal for DATE_TIME.
When entering information into the portal, the record is not committed until you either go to the next record in the portal or click out of the portal. The auto-enter lookup will then take place.
Bingo, thank you.
I have noticed this behavior too (child won't lookup from or through uncommitted parent). Perhaps this should be considered a bug.