[w8] Route-closing

[1.21.6]

I'm cancelling a route due to lack of passenger and excessive competition (Zurich > Belfast Intl)

but this message appears:

Route closure not allowed.
Existing route is the only connection between two or more other routes flown by this plane.

and I can't cancel my route.

This isn't my only route on this plane, the same plane is flying:
Zurich > Belfast Intl (the one im cancelling)
Zurich > Algiers
London/LTN > Pisa
London/LTN > Rome Ciampino
London/LTN > Torino

Maybe this isn't a bug, maybe it is. So I'm posting here. If this isn't a bug, I apologize for that, but please give me a solution.

Thank you

[Wizzie]

If London-Belfast was there this wouldn't be a bug.
Just to make sure are there ANYMORE routes other than the ones posted?

[Cheung Airlines]

Zurich > Belfast Intl (the one im cancelling)
Zurich > Algiers
London/LTN > Pisa
London/LTN > Rome Ciampino
London/LTN > Torino

If you cancel Zurich - Belfast, the plane has to fly empty from London - Algiers / Zurich.
Meanwhile, the hour left on that plane was not enough for her to fly from London to either one of the airport mentioned above, so the route was not able to be cancelled. The only way to cancel ZUR - Belfast is to cancel ZUR - Algiers and ZUR - Belfast at the same time.

I hope I'm right cos I've also experienced similiar situation mentioned above.