How to find who change the Teams Coexistence mode from Teams only to Island mode

sandipk 

As per Microsoft's documentation, it should be included under the Changed organization setting when using the Teams - Audit Logs. 

Friendly Name: Changed organization setting

Operation: TeamsTenantSettingChanged

Description: The TeamsTenantSettingChanged operation is logged when the following activities are performed by a global admin in the Microsoft 365 admin center. These activities affect org-wide Teams settings. To learn more, see https://docs.microsoft.com/en-us/microsoftteams/enable-features-office-365.
For each of these activities, a description of the setting that was changed (shown in parentheses) is displayed in the Item column in the the audit log search results.

- Enables or disables interoperability between Microsoft Teams and Skype for Business for the organization (Skype for Business interoperability).

I tried to test it by changing my interoperability from Islands to SkypeForBusinessWithTeamsCollaboration and search it under Audit Log but nothing shows also. I am not sure if it's propagating or loading on the backend but let me check tomorrow. 

Also, try to expand the time on your search.