Steps to reproduce:

1. Any trigger raise problem event
2. Resolve problem
3. OK event is generated, problem automatically closed
4. Get raised events via API, field c_eventid is NULL

Result:

1. Events in database:
2. Database event_recovery:
3. Call of event.get API result:

       {
         "eventid": "42", 
          "source": "0", 
          "object": "0", 
          "objectid": "16356", 
          "clock": "1574160115", 
          "value": "0", 
          "acknowledged": "0", 
          "ns": "770751216", 
          "name": "Value from test_mes is equal to 1", 
          "severity": "0", 
          "r_eventid": "0", 
          "c_eventid": "0", 
          "correlationid": "0", 
          "userid": "0", 
          "suppressed": "0"
        }
      

Hello.
On our environments field c_eventid is always empty for OK event after resolving any problem.
I've attached what we have in database and what we get by calling event.get API.
By documentation (https://www.zabbix.com/documentation/4.0/ru/manual/api/reference/event/object) we except following:

r_eventid - Recovery event ID
c_eventid - Problem event ID who generated OK event

Value of r_eventid is correct for problem event but c_eventid is always empty.
Also there is no errors in postgresql logs during reproducing this problem.

Could you please advise what is possible cause of this behavior? Is our expectation about system behavior correct? Is any additional configuration needed?

Thank you.