I've seen the same thing. I'm convinced that Riot has a hidden mechanism that intentionally essentially deletes certain champs or classes from particular games. I played one yesterday where I decided to go for 6 knights + Braum (yes I know this isn't a good strat to win, but I wanted to try for funsies). I saved up to 50 gold, and rerolled the heck outta level 5, spending literally >100 gold. Level 5 is still a good chance of rolling a level 1 unit, so I should have been able to shoot for level 3 on one or two of the knights. No one else in the game had any knights at all except one person had a level 2 Mord. Everyone else had a bunch of other tier 1 units, which pulled them out of the pool, increasing my chances of getting knights even more.
After >100 gold on rerolls, I entered level 6 with: 2 Darius's, 1 Garen, 1 Mord, 2 Braums. That's not a level 2 darius, that's 2 level 1 Darius's. I didn't have even a single level 2 out of FOUR champs that I was specifically targeting, and no one else in the entire game had more than a single level 2 of. Tell me that's RNG, and I'll do some calculations about the unbelieveably tiny chance that will ever happen.
And this is FAR from a one off thing. I regularly try targeted strats (again, not seriously or in ranked, just casually to try fun item builds etc). Usually I can find the champs I'm looking for, sometimes more (and get level 3) and sometimes less (only 6-7). But sometimes it will happen again: it's impossible to even get a level 2 of a champ that no one else on the board has, if I can find them even at all. Sometimes it's an entire class that seems to be deleted, and sometimes it's just a single champ, where I'll get Garen and Mord to level 3 while never finding a single Darius in the entire game.
I've theorized that this is intentionally coded into the game in order to force people to adapt, rather than have a pre-set strat. I have no proof beyond trends I've noticed over hundreds of games, and mathematical calculations of how likely those rare instances should occur. I've seen no suggestion or corroboration from official Rioters. But by golly if this isn't the case, I've hit the weirdest consistently statistically impossible patch of RNG imaginable.