This is not an /inc bug. It is just cos mirc limits numbers. The biggest number mirc can handle is $str(9,17).
1 with 17 zeros after it is already too big (18 characters). Any operation you perform in this number will not work
//var %number = $str(9,17)
inc %number
echo -a :: %number (reached biggest number, mirc can't handle operations on it)
inc %number
echo -a inc 1: %number
echo -a +1 : $calc(%number + 1)
echo -a -1 : $calc(%number - 1)
Result:
:: 100000000000000000 (reached biggest number, mirc can't handle operations on it)
inc 1: 100000000000000000
+1 : 100000000000000000
-1 : 100000000000000000
As you can see, you could increase number $str(9,17) with 1, but not with 2. This is cos 1 followed by 18 zeros is the biggest number that mirc can get/store/whatever.
However, $calc(1000000000000000000000000000000000000000000000*5000) has a length of 49, which differs of length of $calc(1000000000000000000000000000000000000000000000) which is 45, so it still performs some operations on bigger numbers, just that it doesn't perform them as it should.
Edit: I guess i was a bit wrong about $str(9,17) cos i see it can't handle $str(2,17) but it can handle $str(9,17)