Yes, that's the way it's supposed to work.

I understand you want incremental control of the cannon, but the problem you run into is that Tamiya electronics use that channel of their electronics to fire the main cannon and MG using an exponential signal (triggered by a rapid full deflection of the stick on that channel).

The only way I know it could work like you want, would be to put the cannon elevation/depression on an extra channel that has a knob control on the transmitter(giving incremental control), but then you'd have a new problem of how to fire the cannon/mg, since you won't be getting any exponential signal easily.

You could remedy that by mixing another channel (i.e. program another channel on a switch to use the cannon channel as a slave, and get the instant full deflection to fire the MG or cannon) with the channel used with the know. Use the new channel as master, and slave the knob channel to it. A switched channel gives 100% (or more if you adjust your travel stops) deflection instantly. An open channel with a 3 position switch would be ideal.

Not sure what radio equipment you are using, but I use a Spektrum DX9 on some of my tanks, and I know it would do this. I use a switch to run my cannon and MG on it, and use a stick to elevate or depress. Whatever you do, don't try shunting the the pot on the elevation stick, as it will fry a newer Spektrum.

As to the self centering stick issue, it would be a good idea to make both sticks self centering. Older Spektrums require a new cam and spring (you'll need to send it in to Horizon for that, as they don't sell parts), or on Gen 2 Spektrum radios (Gen 2 radios have that sexy English accented female voice that tells you "bind complete", etc), simply adjust using the screws under the stick on grip pads.