GraphAPI Create SharePoint ListItem, timestamp format ignored

API ignores 'Z' char - UTC indicator, or explicit zone. If posted as local (without zone info) it is parsed as UTC ... ?!?

I am unable to create a list item with DateTime field. Since documentation neither API did not comment it, I presume that dateTime format should be the same as is returned from server (full iso, e.g.: "Modified": "2021-06-22T02:35:15Z").
Unfortunatelly, the only way how to post a value is strip zone completelly )-:
The problem is in: HTTP API (REST) and java API.

Sample request:

POST https://graph.microsoft.com/v1.0/sites/siteId/lists/listId/items  
Content-Type: application/json  
Authorization: Bearer token  
  
{  
  "fields": {  
    "Title": "any title",  
    "timestamp": "2021-06-22T12:00:00"  
  }  
}  

Sample code:

Instant now = Instant  
             .now()  
             .truncatedTo(ChronoUnit.SECONDS)  
             ;  
     String timestampString = DateTimeFormatter.ISO_INSTANT.format(now);  
     System.out.println("timestampString=" + timestampString);  
  
     FieldValueSet fieldValueSet = new FieldValueSet();  
     fieldValueSet.additionalDataManager().put("timestamp", new JsonPrimitive(timestampString));  
  
     listItem.fields = fieldValueSet;  
  
     ListItem li = graphClient  
             .sites(siteId)  
             .lists(listId)  
             .items()  
             .buildRequest()  
             .post(listItem);  
  
     System.out.println("createdDateTime=" +li.createdDateTime);  
 }  

RESULT LOG:
timestampString=2021-06-22T08:58:02Z
createDateTime= 2021-06-22T01:58:02Z

HTTP requests examples:
Request: "begin": "2000-01-01T00:00:00Z"
Response: "begin": "1999-12-31T16:00:00Z"
-8h

Request: "begin": "2021-06-22T12:00:00+02:00"
Response: "begin": "2021-06-22T03:00:00Z"
-?h

Request: "begin": "2021-06-22T12:00:00Z"
Response: "begin": "2021-06-22T05:00:00Z"
-7h

Request: "begin": "2021-06-22T12:00:00"
Response: "begin": "2021-06-22T12:00:00Z"
Ignores UTC - Z char ...

See also:
https://stackoverflow.com/questions/66283825/microsoft-graph-api-insert-new-listitem-field-datetime-gap-to-8-hours
https://video2.skills-academy.com/en-us/answers/questions/188584/different-time-is-returned-when-creating-sharepoin.html?childToView=446687#answer-446687
AB#9998
https://github.com/microsoftgraph/msgraph-sdk-java/issues/813