This is not really a bug that can be fixed, the meter that medic has when his über is charging is only there because he is medic not because of the medigun so if trying to use it on any other class it will not be there, the kritzkrieg will get über eventually but you just have to guess when you have it but as you said it will not pop since your mouse2 function as heavy using a secondary is not reserved for popping uber, this is similar to if you attempt to use boxing gloves as any class other than heavy the right click will not punch with them and also I don't know why there is no visual effect for when you have über but I'm assuming its due to the same reason that I specified before.
Also just FYI this kind of bug will occur with many combinations from the /gimme command since it is a custom command and is clearly not optimised to go well with the game.
