This has happened to me and others WAY too much to be a "coincidence". (Esp when you are in promos!). I'll use my most recent example. Climbing up silver ladder. Win 11-12 games in a row and end up in promos for Silver 2. The VERY first game we get a ADC ashe player that (swear to god was a BOT) had a level 31 status, UNRANKED, trolled and fed the only games of ranked they had and played a champ with mastery level of 9 points!
Now how the hell did this fool get into a RANKED game for higher silver nearing Gold? Hmmmm o_O.
This isnt the only time I have seen this or had it happen to me as well.