This is to with scoping, data can be recorded at a user, session, item or event level scope.

Mixing the scopes is a bad idea although GA4 does not warn you when it happens. An event can happen many times in a session so its not possible to give an engagement rate because the answer could be both yes and no during the same session. the quick answer is to use segments for session with and without the event.

Its a complicated topic so you will benefit from googling but i have found this page to be really useful
https://www.optimizesmart.com/ga4-scopes-explained-user-session-event-item-scopes/