lukaszek is referring to people "mana weaving" their shuffles irl. I used to see it a lot back in the day, you put a land and two non-lands together, then shuffle. Usually the shuffle is insufficient (otherwise the weave would do nothing) and results in a predictable pattern.
It is considered an offence on tournaments these days, and is a very common reason why people tend to go all wtf over the shuffler (not just arena, saw the same in magic duels)
Clumps of land are expected to form after a random shuffle, even if you're running 18 lands (because of the sheer amount), so you need to deal with this. If you see an aberrant number of non-games, especially alternating between mana screw and mana flood,
it usually means you didn't give yourself enough mana. Better to have one too many than one too few. Be sure to give yourself some draw/scry.
This article discusses the maths behind it:
https://www.channelfireball.com/art...you-need-to-consistently-hit-your-land-drops/
specifically, you want the tl;dr in "Results" table and see what % look comfortable for your deck. From my experience, the arena shuffler provides very similar results.
For an even more tl;dr,
26 lands is for control decks that need a land every turn and have good draw to break floods. You can go down to 24-25 if you use opts, birth of melepetis, etc.
Other than this, the 25 lands setup suggested by the deckbuilder is a trap cause it gives you significant flood chance.
24 is good for decks like UG flash, where you need to reliably go to three and four lands.
22 seems to be a good baseline for two color aggro. I even run it in mono red, because you have plenty of shit to use that mana for (most notably, the castle). Don't go 22 lands with three colors, that's asking for trouble.
Below 22 is good for decks that can operate well on measly 1-2 mana, rarely going to 4 lands. Usually monocolored stuff can pull this off.
The only two decks that could consistently pull off 18-20 lands on Arena was pre-rotation mono red, which was basically filled with Ghitu Lavarunners, Light up the Stages and Wizard's Lightning. The other was mono blue tempo,
which had a full set of shocks and draws cards immediately on turn 2 with curious obsession. You are usually content with 1-2 lands in your hand cause that's all the deck needs, and rely entirely on card draw to get to land number three, which is usually your most expensive shit.
Usually, when I go above 22 lands and I am not using three colors, I would use some utility lands - Mobilized District, Blast Zone, etc., so that not all of the land clumps will be useless.
From practice, if you see you're getting an abnormal land distribution in your starter hand, just mulligan it away. E.g. you're playing a 26 land deck and got two lands to work with,
I would expect the trend to continue and you might miss your third and fourth land drops.
I would typically mull it away, unless I have a scryland. Birth of Melepetis or some draw to dig deeper.
Similarly, when you're on aggro and you see a four land starter, don't assume you now "deserve" to get a streak of all non-lands. Most likely there will be more flooding.
You should reasonably expect 10% of your games to have some sort of mana issues, and it's by design. If you're seeing it more frequently, you may have gotten an unlucky streak, or you dun fuk'd on your lands.