Oh crap... how'd you find that one? At a glance looks like a cache bug, yes. The commit it fixes is 6 months old though, so still more likely to be that Mempool ran the older version.

Maybe mempool was running this patch and blockstream wasn't?

https://github.com/ElementsProject/elements/commit/c26d719c29a40da280a825b25657e9c3d8bc7d99

Scoresby

I suspect that too, yes. But I am fully prepared to be not surprised if it is the other way around! I mean... stranger things have happened.