Thanks for the patch. Unfortunately, it's not a complete fix.

Arithmetic expressions are parsed for substitutions and then, in a second pass, evaluate identifiers into their values. Thus, the incumbent code is correct, and even idiomatic (consider foo=2+2; echo $((foo*2)) $(($foo*2))), and conversely, the patch would seem to merely paper over the issue.

I'm curious why it works, but we can discuss that back on the issue's ticket.