Weird: `cache-control` header ignored for one route only

All my routes respect s-maxage=300 except /products/[slug], which always shows x-drift-cache: MISS. Same handler code. What can make a single route uncacheable?

Does that route read cookies or headers? Any dynamic API call opts the whole route out of the cache.

It reads a currency cookie... that would do it. Moving that to the client.