Well, that's it for Vettel's second second place this season.
Apparently the car sprung a minor fuel leak and lost 1.3 liters of fuel over the race, which never showed up in the in-out accounting between start weight and injected fuel measurements.
I don't see how the penalty fits the violation here - but I guess that's the price for having a reasonably simple ruleset.
Still, some technical steward leeway for first-time, "explainable" violations, which are due to issues that have a limited benefit to the team (the leak could have gained Vettel time over the race, but he was stuck behind Ocon anyway, so any benefit was probably lost in ultimately unsuccessful attacks), and are evidently not intentional (hence why there's the one-time/first-time qualifier), would not looks out of place.
Compared to the inconsistent penalties for driving infringements, which to the audience have a much more noticeable impact, I don't see how this argument could not be made.
Particularly in this time of a cost-cap, I don't see how the leniency on first-time infringements could sensibly be exploited, by a "do it once for marginal gains, where it matters" mindset. In this case for example, I don't even think they were underweight, as the penalty was for fuel sampling only. So the only real benefit would be, that the fuel would have been tuned in such a marginal way, for this one race, that the illegal additive would have been impossible to detect in the reduced fuel sample, all the while being sufficiently well tested to be beneficial across the race distance. Given the engine tuning and validation required to do this change for one race only, I believe the motive can be easily discarded.
So in the spirit of the law, I see no reason to penalize.....