See the attached data files captured with a backup cursor + duplicate backup cursor.

The backup cursor copies a checkpoint with the following properties:

{
  "oplogStart" : { "ts" : Timestamp(1585249255, 1), "t" : NumberLong(1) }, 
  "oplogEnd" : { "ts" : Timestamp(1585249275, 1), "t" : NumberLong(1) }, 
  "checkpointTimestamp" : Timestamp(1585249255, 1) 
}

Note the oplogEnd is not expected to be in the checkpoint, but rather the journal file ./data_tmp_backup/db1/journal/WiredTigerLog.0000000001

For kicks, I also added more data and duplicated the backup cursor (./data_tmp_backup/db1/journal/WiredTigerLog.0000000002 in the attached data files). We extended to Timestamp(1585249400, 1). A working binary (master/v4.2) starting up on the attached data files finds this as timestamp as the last oplog entry: "ts" : Timestamp(1585249468, 3).

However, using the tip of v4.4 (5599378dc4137896c87ec7104a9b1fe9917f9795), the last oplog entry matches the recovery/checkpoint timestamp:

{"t":{"$date":"2020-03-26T15:16:07.215-04:00"},"s":"I", "c":"STORAGE", "id":22430,"ctx":"initandlisten","msg":"WiredTiger message {message}","attr":{"message":"[1585250167:215211][15142:0x7f53c94f9a80], txn-recover: Set global recovery timestamp: (1585249255, 1)"}}

test> oplog.lfindOne()
{
	"ts" : Timestamp(1585249255, 1),
	"t" : NumberLong(1),
	"h" : NumberLong(0),
	"v" : 2,
	"op" : "n",
	"ns" : "",
	"wall" : ISODate("2020-03-26T19:00:55.364Z"),
	"o" : {
		"msg" : "Reconfig set",
		"version" : 2
	}
}

In the attached datafiles, the oplog is WT table collection-16--1190650592073264752. which is WT fileid 20.