Yeah that was similar to my experience.
I was plat 3 end of season 6, was eventually placed silver 1 in flex queue. Getting like 27ish LP per win makes me feel like I ouaght to have placed higher (not to mention being on like a 7 game win streak) so that's a bit odd. Pretty much all of my flex queue games during my promo series to gold 5 flex have consisted of high plats and low diamonds, who when I look them up are like bronze 1 in flex, which confuses me too.
If it was matchmaking based on last years solo queue mmr I'd expect to hear about 'fair' matches, but people like you (and even me since I should in theory be facing mid plats not high plats) seem to contradict that. If it was based on a flex queue mmr, I'd expect to see everyone have about the same flex queue rank, but that seems to have full tier differences too.
It's weird. Hopefully solo/duo queue will make a bit more sense when it releases.