Yes, I believe the round up happens at the point of original posting but the transaction then settles a few days later at a different value making the original roundup ‘incorrect’ in the strictest terms.
It’s the way it’s designed to work at the moment, simply because foreign currency transactions are a special case that hasn’t been accounted for. From memory, (so it could be wrong) changing this isn’t straightforward so isn’t likely to happen any time soon, if at all.