See the attached data files captured with a backup cursor + duplicate backup cursor.
The backup cursor copies a checkpoint with the following properties:
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:
In the attached datafiles, the oplog is WT table collection-16--1190650592073264752. which is WT fileid 20.