I have recently activate a free trial Azure accounts with a trial P2 license then setup the Azure Sentinel. I have activated the Azure Active Directory and the Azure Activity connectors through the data connector page.
Audit logs, Usage and Azure Activity have been flow into the Sentinel successfully however no Signin logs have been seen so far. I have gone through the prerequisites in the connecting Azure AD to the Sentinel link (https://docs.microsoft.com/en-us/azure/sentinel/connect-azure-active-directory) quite a few times. I have given myself a role of Contributor, Azure Sentinel Contributor, and Global admin, still no luck.
I have checked there are few forums have been talking about this:
https://techcommunity.microsoft.com/t5/azure-active-directory-identity/signinlogs-are-not-showing-in-log-analytics-azure-monitor/m-p/1692381
https://techcommunity.microsoft.com/t5/azure-monitor/unable-to-view-signin-logs-in-log-analytics-workspace/m-p/895691