Hi folks!
Not sure if anyone else encountered the aforementioned problem, but in any case and for future reference (hopefully not for long), keep a lookout if you're trying to push a non-integer Value to the /Client/UpdatePlayerStatistics PlayFab endpoint - it seems that the API will reject it with a not-so-helpful error message along the lines of:
Error:Invalid input parameters Statistics[0].StatisticName: The StatisticName field is required.
Since my crappy last minute game-prototype-thingy was intended to be speedrun, I was trying to submit fractional numbers (seconds) and had no idea this was an issue, because the docs said the allowed type is "number", not "integer". I've found the clue and solution in the PlayFab community forums - after the deadline of course ;) Just my luck, I should've googled earlier.
I've already fixed the bug, wrote a bit about it in the devlog and will update the game if I can; I hope the info will be useful for others as well.