I don't think this is a bug. Non-static local variables are reset to their initial value when they are "run" (i.e. imagine an "every tick" event in the same position resetting the variable). So first of all a local variable is not suitable for a countdown that needs to remember its old value. Secondly that message is just a debug note. I've removed it for the next build since it doesn't really matter. It's a bit unusual to access local variables from a trigger, because local variables only really "exist" for the normal run of evaluated events within their group - and triggers fire outside of that. So it's taking a "this variable doesn't exist" code path (which previously logged a debug message), and falling back to returning the initial value.
Making the variable static also works, since that stops it resetting every tick and makes its lifetime permanent.