Are some of the downstairs lounge accessible bathrooms ever really blocked off? Doesn't make sense.
Sometimes on long trips the retention tanks may be full or there may not be anough water for flushing or washing hands, so to avoid passengers entering an uncomfortable situation, it may be the best thing under the circumstances to lock the affected restromms as long as there are still alternative restrooms available elsewhere on the train.
It may also be that if attendants discover that a restroom is dirty and needs to be cleaned, but they don't immediately have time or other things have higher priority, that they temporarily lock that restroom until they have time to attend to it. Again this is for the passenger's comfort as they don't want to
find a dirty restroom.