I don't have a 590 but logically it should be able to cope.
You don't say how you are adding the fuel station to the route you are running but the owner's manual says:
Adding a Point to a Route
Before you can add a point, you must be navigating a route.
1 From the map, select > Where To?.
2 Search for a location.
3 Select a location.
4 SelectGo!.
5 Select Add to Active Route.
Can you confirm this is the way you are doing it, please.
The manual can be found here:
http://www.activegps.co.uk/user-manuals/garmin-zumo-590lm-manual.pdf page 9 of 22
On the same page, the manual says that the 590 has an 'Up ahead' function. Have you tried selecting a fuel station and adding it to a route that way?
Failing all / any of the above, here's a suggested workaround:
Find the fuel station you want to ride to.
Save it as a favourite.
Stop the route you are running.
Select 'Where to' and then favourites.
The fuel station, assuming it is close to you, should be at or near the top of the list.
Select 'Go to' the device will create you a route to the fuel station
based on your routing preferences; if you have it send to windy roads, avoiding motorways and you really want to get there fast, you'll probably be very annoyed at the device's stupidity. It's not its fault!
Ride to the fuel station and fill up. Then restart your route from that point. Take care though as, depending on how you set your route up (and maybe depending on your pre-set preferences and if you have any specific waypoints in it) the device might alter your route. Check before riding off. If it has, do not panic. Stop the route. Select it again but do not run it. Simply navigate to the magenta line, using the device as a virtual paper map. It may well not scroll, so you'll need to drag the map around and / or zoom in and out as you ride. When you hit the magenta line, start and run the route as normal, just checking that you are facing in the right direction!
PS One thought or guess. If the route you are running has lots of waypoints (points you have told the device you MUST go through) in it and you then add in a fuel station as another waypoint and / or if you are already some way off your route, the device might well be struggling to plot you something,
particularly if the fuel station you have chosen is behind you.