I believe this is a bug. It happens where the origin and departure airports (which could be the same airport) share runway number prefixes, e.g. EGLL/09L and EHAM/09.
- Create a new flight using manual input from EGLL to EHAM, do not auto-generate route.
- Select runway 09L or 09R as the departure runway
- Select direct to runway 09 at the destination.
- Select “Type Route” to edit the text version of the route, noting the format of the destination.
- Press “Save”.
Error dialog appears suggesting that origin or destination airport has changed. This appears to be because the destination is EHAM-09 rather than EHAM/09. Switching to a different runway, e.g. 22 reverts to the accepted format of the destination, i.e. EHAM/22, and the problem disappears.
It occurs where there is no suffix on the runway number too, e.g. EGGP/27 to EHAM/27. It’s only an issue when editing the route text by hand, because of the format of the destination, e.g. EHAM-27 rather than EHAM/27. Changing the “-” to “/” in the route text allows the route to be saved.
It’s a reasonably obscure one, and has a workaround, but still a bug I think.
EDIT: this doesn’t happen on the iOS app but it does happen on the Mac OS desktop app and through the website.