Got the same issue now on my previously best paying route, which has a demand for 2800 passengers and no competition.
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hmmm what route is that (please tell me the airport code)? any changes to quality or ticket price?
It looks a bit different from the previous bug that it usually jump back to 100% afterwards, yours seem like a pretty consistent decline
Take note that direct competition is only one thing. There are many other factors, for example (and not limited to)
1. Direct demand can be fulfilled by other routes that is not shown as direct competitor. For example if u have a route from new york to los angeles other airlines can steal ur passenger by offering a route from new york to las vegas and from las vegas to LA. It wouldn't be shown as direct competitor but it can still compete with yours. Direct flight is more attractive to passengers but if the price is low enough, passengers will be taking connection flight
2. Your existing route might be benefiting by other connection flights before (maybe even offered by other airlines). Take the same new york to LA example, if there's another major airline that used to fly from London to new york, then it would have boosted your JFK <-> LAX route too, as passenger were traveling from London to new york and then from new york to LA. Now if that airline raises price/reduce frequency/cease the service, the demand for your route will drop as well
Above are just 2 simple examples, there are probably other scenarios as well. Study the passenger map to see the flow of passengers.
You can also gimme the route airports and i can check the reason