Blank details for user when attempting to send email

Hello Team,

Below issue noticed intermittently for one user while attempting to send email 

Whenever user A attempts to send email or meeting requests to user B in same domain, below issue is noticed.

However ,  issue not observed whenever user attempts to pick the user from GAL 

We have removed the entry from NK2 . Enabled online mode and enabled cached exchange mode

Issue reoccurs after few days.

Today when issue reported , we checked and found that the said issue could not be reproduced.

Both the users are in office 365 in same domain.

Issue has not occurred in OWA.