The WT_STAT_SESSION_TXN_BYTES_DIRTY statistic appears to be overcounting compared to other cache statistics such as cache_bytes_updates.

The reason is that the statistic is updated in wt_cache_page_inmem_incr without checking the page type so it is also counting changes to internal pages. That puts the value out of line with cache_bytes_updates.

The fix is that the session statistic increment should be put under the if (!WT_PAGE_IS_INTERNAL(page) && !btree->lsm_primary) conditional just below its existing line.