You can fly manually under anything that cuts off the GPS signal PROVIDED you do NOT lose the Wifi connection. If that happened, the Anafi would go into RTH mode and fly up to whatever the RTH altitude was set at, probably crashing into the the bridge. The way to ensure that you don't lose Wifi is to follow the drone under the obstruction and keep it within 50 to 100 feet. Wifi signal strength may be poor under the obstruction and that is the reason to follow it so closely. The height of the bridge and the width of it will have a great deal to do with the success or failure. For instance, the Golden Gate is four lanes wide, but it's 220 feet high....you could fly your Anafi under it from your boat with no problems, or even from the beach at the end. Trying to fly under a four lane bridge that is only 20 feet high might not work unless you could walk under it or at least keep the drone in good line of sight until it got out the other side.
As I understand it, when using a Flight Plan losing Wifi connection with the Anafi is not a problem as it will continue to fly the Plan to the end. However, losing GPS at the drone will cause it to abandon the Flight Plan and initiate RTH and crash into any overhead obstruction unless it is above the RTH altitude.....which you can set as low as 20 feet if I recall.
I would test this with a small bridge before trying a large bridge!