Did you duo with anyone? (Or more than duo if we're talking flex?)
You can really bias your mmr gains/losses compared to solo by duoing with someone. I've heard of fresh accounts getting placed plat 5 when duoing with their high mmr'd gold friends, while if you duo with a bronze 5 mmr'd person you'd have negligible mmr gains even for all 10 wins.
Although I'm rather surprised that you're below silver 4, as I thought that was where everyone started mmr-wise.
While it's never placed before, has it played ranked matches before (e.g. <10 matches in a previous season so as to remain unplaced)?
In theory it's your final mmr that is the only thing that matters, not your actual W/L record, so occasionally people trying to carry easy games with a low rank friend end up shooting themselves in the foot.