Summary:
WiredTiger can return the wrong value when performing multiple timestamp based searches using the same WT_SESSION handle.

This was originally started in WT-5119, but I believe the set of recent mismatch failures are a different problem than the original one in WT-5119. That ticket deals with an empty value that appears while repeating within the same transaction, and this ticket deals with modify records (so far).

The original failure was on margay-aws: http://build.wiredtiger.com:8080/job/wiredtiger-test-format-stress/71695/

Another on tinderbox:
http://build.wiredtiger.com:8080/job/wiredtiger-test-format-stress-sanitizer-old-branches/1715/

And a third on kodkod-aws:
http://build.wiredtiger.com:8080/job/wiredtiger-test-format-stress-sanitizer/24917/

I will copy some of the analysis content from WT-5119 to this ticket for completeness.