Apart from the shady "randomizer" that gives you too many duplets and triples of cards and too frequent wonky mana games (if I keep a hand with 4 lands, 95% of the time I will draw another land), I have a feeling that my decks are matched against particular archetypes. Like, my mono-black discard decks usual opponent is merfolk. At this point it seems to be ~40-50% of my matchups.

Mono-red aggro is by far the most common opponent when I play my mono-green stompy.

Mind, I haven't taken notes so this is mostly my estimation.

And your data is?

The randomisation is normal and works perfectly to the best of any computer's capability but then that is the problem; it is more random than paper because people don't shuffle well enough to get close to random, plus the Bo1 games make people see way more games and people also remember all the bad times and weird patterns more.

Can't say anything about you matching certain archetypes but if you've not recorded all your games and don't have proper data to show it then its hard to say if perhaps its just more to do with humans being good at noticing patterns.
There is some amount of rigging in the match maker to try and match jankier decks with other janky decks and tier 1s with other tier 1s but its not clear how it checks, possibly just rarity level, also its just the ladder and not constructed event.

Yeah, even as I think flood-out at a ridiculously high rate, I'm still pretty sure it's just a combination of sample size and my brain focusing on the bad games vs. the good games.

And I'd kill for a meta filled with Merfolk lol. Are you playing Constructed Events? It may be a situation where you sit pretty low on the ladder for quick play matchups and merfolk is just an easy-to-assemble deck from the starter decks.

All of this

Playing best of 1 ladder mostly.

The matchmaking algorithm on bo1 ladder will pit you against the same decks often, yes. Also, improbable mirrors when playing niche or meme decks...
That's not rigged though, it's just an ill-conceived feature.

Play other modes.

It is easy to believe it's rigged. I too suffer from chronic confirmation bias when it comes to flood/screw.

I'm aware of confirmation bias, but still believe that card grouping is real in Arena.

Card grouping is actually a sign of randomness. If no two alike cards were ever next to each other, that would be a predictable pattern.

That's not what I'm saying. But without data, everything is just anecdotes and feelings.

Btw, keep a hand with 4 lands and count the times you will draw land #5 as your first draw. I feel like it happens like 90% of the time I do it.

Oh, and the pass turn button still doesn't work. White life trigger decks take forever to play against with their gazillion triggers I must resolve.

It does seen weird sometimes. I would keep track but don't want to get sucked in. Clumping, Screw, Flood happens to me also in what seems like an inordinate amount of times but I'm pretty sure I'm just seeing it that way without hard data to back it up.
One thing with data I can say though is I speak with GoA almost every day and he has complained more than once about the "randomness" as to weather you or on the draw of play and I've know I've had those days also.
Today he counted and had 19 opponents go first and 4 you go firsts. That seems rather unfair even if it's truly random.