As part of WT-9186, wt_txn_validate_commit_timestamp is changing the way it validates the commit timestamp. Now that we don't have out-of-order timestamps, we can (correctly) compare the commit timestamp of the first update on the update chain with the stable timestamp, instead of the commit timestamp from (say) session->timestamp_transaction("commit_timestamp=...").

There seems to be a non-zero amount of server fallout from this change, I've linked a patch build in the comments. This will need to be investigated/fixed before we can include this change in WT.