Do an in game bug request. Explain that in the message area, if its true we want it on the list.

Thanks.

You're not the only one to report this, but having tried it myself I did not encounter the issue so there is no need for a bug report, this forum post will be enough, to see if others have the issue

It's not a bug !!

What i did and assume most do, is to send your troops into the city you want to conquer.
But select your army and then target the airplane. It will then fight this aircraft, and when you win you have the city/province.

Took me a while to figure out too.

While its not necessarily a bug, we should change the behavior I think.
Flying airplanes should not count as 'garrison' of the province.