I upgraded from 7.7.3 to 7.7.6 in a project recently, search for customer email in sales works for me.
Not that big version upgrade, but thought it worth mentioning that it works for us in 7.7.6 as well.
The search will always try to match if your search phrase starts with the configured order number prefix.
If you have set it to empty, like we have in our solution, the search will always assume you are trying search by ordernumber, and you will not get any hits on for example email or customer name.