btree->last_recno gets updated in col_append_serial_func, where we're adding a new entry to the end of the btree and so might need to bump last_recno.

/*
 * Set the calling cursor's record number. If we extended the file, update the last record
 * number.
 */
*recnop = recno;
if (recno > btree->last_recno)
    btree->last_recno = recno;

The trouble here is that the btree could be accessed or updated by multiple threads (it's a property of the dhandle), so concurrent appends could leave it in a bad state. For example, two threads have a local recno greater than btree->recno, attempt to update it, and the smaller value wins. This would result in data corruption since we would reuse an old record number the next time we append.