@Maurice L , Thank you for reaching out. Unfortunately, we do not have the option to collect signing logs for the Client-Credentials flow. Client Credential Flow is referred to as a non-interactive flow and currently, we are only registering the interactive flows in Azure AD signing logs.
But the good news is, we are getting this feature of registering sign-in logs for non-interactive login also pretty soon. You can find more details here: https://feedback.azure.com/forums/169401-azure-active-directory/suggestions/31991029-make-spn-non-interactive-login-events-logged-and
Hope this helps.
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