I get this issue a LOT.
Basically, some nights you’ll log in and get in one game after another and if the map isn’t king of the hill, you’ll almost exclusively be a defender.
There are a lot of people complaining about exactly the same issue.
https://us.forums.blizzard.com/en/overwatch/search?q=only%20defense%20games
I suspect the issue is that people who queue up in groups that consist of just tanks/heals get an actual random assortment and that anyone who queues up solo (or with others that are taking dps roles) gets shafted.
My suspicion that the role queue is the culprit springs from the fact that this issue started the day it was implemented.
Before role queue, I would get Rialta on Attack, the Rialta on Defense, then Gibraltar on Defense, then Gibraltar on Attack, then I would have all 11 recent players switch out and get a random assignation into another scroll of games.