See this code. It currently uses WT_ORDERED_READ(prepare_state, upd->prepare_state) for both the first and second read. However, it puts the load fence after the load, while the second read of prepare_state needs the fence before the load. In particular, it needs to be ordered with the loads of upd->start_ts and upd->durable_ts on line 943. For completeness, here are the store barriers that those read barriers would be pairing with.