Mono/stereo output and fx

I'm trying to figure out how this is supposed to work: when you have a mono vocal track, and add a mono effect to the FX Chain in the ProChannel, you cannot change the output of the channel to stereo anymore (for intstance when you decide to add a stereo effect to the chain). But when you change the output to stereo first, you can add both mono ánd stereo fx (that end up as stereo output too). You can switch back to mono output with a stereo effect in the FX Chain without any problems, so it doesn't feel very consistent.
 
Is this a bug or intended design? I'd say that it shouldn't matter if you have added mono fx: you should be able to change the output to stereo at any time?