Customers who have received replacement AirPods have found that they can't actually use them because they've been shipped with a version of firmware that hasn't actually been released yet.
Second gen AirPods are shipping with the unreleased 2D3 firmware, which appears to have been happening since March based on the complaints on the MacRumors forum and over on Reddit. Those who have received a replacement with the 2D3 firmware can't use it with their existing earbud, which is understandably ticking them off. It's not like you can just update the one you already have to the same firmware because it's not been publicly released.
Some people are reporting that it's predominantly the right earbud replacements that are experiencing this issue, while the OP on the Reddit thread about it says they've spoken to Apple Support but there's no internal documentation for the technicians to use to resolve the issue - because it's unreleased. So basically, it looks like you'll have to just keep flagging the issue, getting more replacements until you find a matching pair with the same firmware.
Ideally, Apple will roll out 2D3 so customers can update their AirPods to get a fully functioning pair, but the company hasn't commented on the situation yet. [iMore]