GMAT Sentence Correction (SC) | Re: Many policy experts say that shifting a portion of

The GMAT will often add commas in places where they aren't strictly necessary, usually to set off a modifier or add clarity to the sentence. Don't look for absolute rules there--you're not likely to find them!

With that in mind, I can see why you'd expect ", but" to follow an independent clause. However, in this case, the GMAT has added a comma to "reset" and nudge us to look back and find parallelism. The sentence would read fine without that comma, but we don't want to rule
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