Honestly, I struggle to see anything wrong - I've had tests like this one which I've been using for ages. I just ran it on my HTC 10 and it consistently measures it hitting v-sync within 1ms. On my desktop it measures <= 0.1ms. In Firefox it's so accurate, the v-sync jank is locked on 0ms. So I can't see anything wrong.
I would guess it's somehow device or OS specific. But so long as everything looks fine on my end, what do you expect me to do? It looks like our engine is v-syncing just fine. As ever, a minimal reproduction with the right hardware and OS combination is the best way to investigate any issue. You can also report issues to browser vendors yourself directly, which I would encourage you to do so - if you report them to me all I would do is forward the report to them, but I won't if I can't reproduce it myself. You can easily cut me out of that process.
If you have general performance concerns, the C2 engine is very mature by now, having been used widely for ~6 years. The performance characteristics are generally excellent, there are no significant known bottlenecks or weaknesses, and it's robust and proven for a wide range of games. I have tested and profiled heavyweight games like The Next Penelope and reviewed our engine's performance, and everything was working fine. In the past, it's also been very, very common that people blame us or browsers for their own mistakes, like extreme fillrate abuse. I am very confident our engine is actually working great. That's not to say there aren't any bugs or issues, but by now finding any kind of serious deficiency in our engine is actually a rare and pretty surprising case. I think you should weigh up the likelihood of that accordingly when running in to issues.