Hi @michael.tran
This might happen because there would be atleast one event recorded for that user wherein the email property didn’t get captured. User properties under the user profile reflect the last captured value of the properties, hence you are seeing the email value in there.
You will have to chart one specific user id in an event segmentation chart and use a group by on email property to see when this (none) event occurred. Hope this helps.
Thank you @Saish Redkar for the explanation. This helps understand the functionality. and I’ll need to explore some more.
A feature enhancement request could be to add an event date qualifier for the Cohort and User-Lookup reporting views... since a custom fields won’t initially have values for users. The existing nuance you mentioned can create friction and require further exploration to obtain accurate reporting.