The logs currently show that the application was updated but do not log the granularity on the individual OUs that were changed. The Event Type application.lifecycle.update will show when an application in Okta was updated. When an OU is de-selected, the next import will update Okta. You can match the prior application updates to the import that removed the users/groups. Below is the system log entry for an OU change I just made to my AD integration.
Time - 10:44:02 Event Type - application.lifecycle.update Message - Update application Outcome - success Actor - Marc Johnston Targets - Active Directory