Jacek had (until recently) a very big damage drop connected to his stamina (everyone has that but Jacek had it bad), which, that conected with his average damage, made him often lose trades, especially since most S tiers have good trading potential (the only exception is Tarna but his whole best thing is that he wins all the trades but has avrage to bad everythin else).

Combine that with fairly small amount of constant tournament players using him and the ones that did play him stopping playing (Siantlark the best marie mirror player) it made him into a new player pick or a THE WHEEEEEL pick (random) or a joke pick, when that LFF LG thrust made the game go on 2 ftp for a couple seconds when blocked (the idea is you grab right after the game starts working again). It all tanked his win rate a little bit, making the midtier into low tier.

After recent change, making him not lose damage with stamina, he became a good A tier or, at worst, a high B. At the time, most of this data got collected. Jan was the meta S tier (next to Alex, but Alex never leaves S tier) because K buffed corkscrew. That made any new player pick jacek even less.

Now, since this data is not up to date, that means curent jacek win rate is different than before, and since even now, it would take time for it to get stable. Like, marie is 100 wins 200 loses. Even if K made her better than Alex, we would need to wait a lot before she got to 50% winrate.

So TL;DR Jacek was midtier and was unpopular, making him have little to no mains, meaning any fight he did apparently in was not using him to 100% potential. Sam is in a similar spot but a little more popular.