Testing routing on the livemap doesn't use that side road, so it can be hard to tell.
One possibility is that, shortly before you came that way, another driver pulled over at the pub. If they weren't routing to
the pub, then the combination of poor phone GPS and Waze thinking they would drive past may have left them "snapped" to the main road. After a minute or two of sitting still with Waze on, the servers probably decided there was traffic on the main road and tried routing you through the side road.
As an editor, the main thing to check is turn restrictions. Click on each segment in turn and you will see green/red arrows indicating allowed/restricted turns. There aren't any here, so that's not a problem. Shift-Z will show all restricted
turns - a quick way to check if there's a problem.
Given that there's nothing preventing proper routing, it's time to try & prevent improper
routing. If that service road is as you describe it, then the solution is fairly simply - change the road type to Parking Lot Road. Waze should never try to route you through
a PLR segment - so this will solve your original problem. The only time routing will use this segment is if the destination is closest to it. An additional advantage is that, if
the GPS fix snaps you to the PLR, no traffic jam will be generated.
You can do this edit yourself, or I will do it for you if you like. There's much more information on the Wiki (link in my signature).