UserLoggedIn events not found in Azure Audit log for about a week

First of all, for login events best use the Azure AD sign-in logs directly, as the unified log often displays them with delay (if at all): https://docs.microsoft.com/en-us/azure/active-directory/reports-monitoring/concept-sign-ins

Second, just because the users access a given application it doesn't mean they do a full-blown login. The application can reuse an already issued refresh token, and until its validity expires, you will not see any login events for the given user/app combo. But 7 days is long enough period to have at least few users try an app they haven't logged in to in a while, so it seems a bit suspicions and most likely the unified log is acting up again. Which brings us back to my previous point, check the AAD logs.