Hey @sebastianFromGrowthPod
You need to ensure that your server side calls are sent with the user id or device id at least so that Amplitude can identify and tie these events to the proper user.
For the sessions, you will have to pass in the sessionId for your server-side calls if you want them to be included in a session. If there isn’t a session id, then these calls will appear as out of session events.
Mostly this is applicable to events sent via HTTP API and I’m not familiar of the webhook way.
You can read more on sessions here- https://help.amplitude.com/hc/en-us/articles/115002323627#h_e0307573-ebb5-4712-ad6b-0bb4c8cc6966
Here’s another post discussing sending session ID with server side events
Hope this helps.