A WT Patch build hit a failure in test_bug019 that appears to be the same issue described in WT-6353. In that ticket, the bug couldn't be reproduced, so sue.loverso added debugging information to the test in the hopes of better diagnosing the next failure.  
The failure shown there is:
[2020/07/24 22:44:54.911] ====================================================================== [2020/07/24 22:44:54.911] FAIL: test_bug019.test_bug019.test_bug019 (subunit.RemotedTestCase) [2020/07/24 22:44:54.911] test_bug019.test_bug019.test_bug019 [2020/07/24 22:44:54.911] ---------------------------------------------------------------------- [2020/07/24 22:44:54.911] _StringException: Traceback (most recent call last): [2020/07/24 22:44:54.911] File "/data/mci/ad6ebb7f5875fa85805c17b220c1ee40/wiredtiger/test/suite/test_bug019.py", line 86, in test_bug019 [2020/07/24 22:44:54.911] self.assertTrue(self.max_prealloc > start_prealloc) [2020/07/24 22:44:54.911] File "/opt/mongodbtoolchain/revisions/e5348beb43e147b74a40f4ca5fb05a330ea646cf/stow/python2-v3.OwH/lib/python2.7/unittest/case.py", line 422, in assertTrue [2020/07/24 22:44:54.911] raise self.failureException(msg) [2020/07/24 22:44:54.911] AssertionError: False is not true [2020/07/24 22:44:54.911] ----------------------------------------------------------------------
Here are the mentions of test_bug019 in the results output of that run of the python tests:
    test_bug019.test_bug019.test_bug019: <wttest.TestSuiteConnection object at 0x7a05887228d0>
    test_bug019.test_bug019.test_bug019: Updating max_prealloc from 1
    test_bug019.test_bug019.test_bug019:     to new prealloc 2
    test_bug019.test_bug019.test_bug019: FAILURE: max_prealloc 2
    test_bug019.test_bug019.test_bug019: FAILURE: start_prealloc 2
    test_bug019.test_bug019.test_bug019: finishing
    test_bug019.test_bug019.test_bug019: passed=False
    test_bug019.test_bug019.test_bug019: preserving directory WT_TEST/test_bug019.test_bug019.test_bug019.0
- is related to
 - 
                    
WT-6353 Print out failing values and value history of prealloc
-         
 - Closed
 
 -