It’s quite a complex query that is fired to retrieve the results.
I’m not sure if the value is empty because the SQL is corrupt, or if there is something else happening.
Are you able to do some debugging or are you a developer?
Could you install the Query Monitor plugin and see if there are any Database errors on the page? If not, can you find the query in the list that is fired to retrieve the total revenue? You could search for strings like ‘woocommerce_order_itemmeta wc_oim2’ to find the corresponding query. Please send any query related info to firstname.lastname@example.org so I can help you further by email.