Google analytics not assigning to traffic acquisition

I'm facing a problem with custom events sent to Google Analytics. The main problem is that custom sent events are being assigned only to user acquisition and not to traffic acquisition.

I'm sending 2 custom events.

1st event starts the session:

{     "client_id": "435384.2118427612",     "events": [         {             "name": "session_start_test",             "params": {                 "session_id": 1733217977,                 "source": "Test source",                 "medium": "cpc",                 "session_number": 1             },             "timestamp_micros": 1733217976999990         }     ] } 

2nd event send source data:

{     "client_id": "435384.2118427612",     "events": [         {             "name": "custom_event_test",             "params": {                 "session_id": 1733217977,                 "engagement_time_msec": 100,                 "title": "Test title",                 "country": "Estonia",                 "source": "Test source",                 "medium": "cpc",                 "campaign": "test",                 "campaign_source": "Test source",                 "campaign_medium": "cpc",                 "session_number": 2             },             "timestamp_micros": 1733217977000000         }     ],     "user_properties": {         "country": {             "value": "Estonia"         }     } } 

Both events are sent to GA and can be seen inside realtime reporting.

The problem that both of these events still assign source/medium to user acquisition instead of traffic acquisition. I've also tried to send utm_source and utm_medium, but that did not work out. Any ideas where is the problem?

I'm sending both events with PHP curl, using Google Measurement Protocol API.