The txn_pinned_timestamp_oldest statistic in src/txn/txn.c is calculated as durable_timestamp - oldest_timestamp (line 2408). Since both timestamps are unsigned uint64_t, when durable_timestamp is less than oldest_timestamp, the subtraction underflows and wraps to a very large positive number. Although this value is subsequently cast to int64_t and clamped to 0 in the statistics aggregation phase, the unsigned underflow is still buggy and should be prevented.

For ReProduce:
A csuite test (wt<this wt number(I added the WT number later, after this issue was opened>_txn_pinned_timestamp_stat) reproduces the bug and verifies the fix (I attached the before/after outputs on JIRA).

Purposed Fix:
Adding a check matching the adjacent lag statistics to ensure the statistic is explicitly set to 0 when durable_timestamp <= oldest_timestamp.