This issue is actually the reason why the "Start a G2G thread" feature was added in the first place.
When proposing or approving a merge, if you're in a position to complete it, you go straight to the completion page. If you bail ot at that point, your approval is recorded. But if you can't get to the completion page because of pre-1500, nothing is recorded.
This means you can only propose or approve a pre-1500 merge without a badge if it needs another approval, and doesn't already have it.
There was a thread about it, but no way I'd find it. Way back soon after pre-1500 started.