The expression to apply for saving is more of a way to get things into the query system for persistence, so it's mainly meant for: you set it, you use it to execute a query, then you discard the entity. If you keep it around, then you have to clear it manually, as it is what David said: it's ambiguous what to do: clear it always or leave it as is. For its intended use case it's discarded anyway, so we don't clear things for you.
The value in the field is actually ignored when expression to apply is set, setting the field doesn't trigger logic to reset that expression.