This is not a bug as much as it is an ancient functionality of /var
the first param of /var must begin with a %, not a $
Secondly, the correct way to write it with a var is to "ignore" the '=' rule and just do:
//var %v $+ b 1 | echo -a %vb
Echo's 1
Note that I didn't use = there. Yes, it's not the documented way to use var, but it works. It's kind of a minor bug I suppose, but it's really been around forever, since the 5.6 days iirc. You just gotta get used to breaking some rules when dealing with mirc script :P