Two separate reasons are stacking here, and it's worth separating them.
The production-side reason: the studio made a deliberate call to keep the episode count small rather than pad the cour, because Frieren's whole appeal sits in long silences and clean, uncompromised key animation — an episode of this show made in a rush doesn't read the same. Ten tight episodes beats twenty-four with visible corner-cutting.
The source-side reason: the manga itself has gone through repeated hiatuses tied to the creators' health, so its release schedule isn't steady enough for the studio to safely commit to a full 24–26 episode order without risking catching up to, or passing, the source. Season 1's 28 episodes had years of banked manga chapters behind it; season 2 doesn't have that same cushion yet.
So no, it's not that the Mimic material got cut or rushed — it's that this cour is deliberately shorter, and the better assumption is more seasons later rather than a compressed adaptation now. (最後まで, "to the very end," is basically the attitude the studio's taking toward not compromising the pacing — they'd rather go slow than go sloppy.)
Been watching long enough to remember when a "short second season" usually just meant the studio ran out of money, so it's worth saying this one's different - it was a deliberate call, not a budget problem.
Two things stacked together, if I've got this right. One, the manga's had repeated hiatuses because of the creators' health, so the source material isn't coming out on a schedule reliable enough for the studio to safely commit to a full 24-26 episode cour the way season 1 got. Two, and this is the one I actually buy more, the production team apparently decided Frieren's whole appeal lives in those long quiet shots and unrushed key animation, and padding a cour just to hit a "normal" episode count would mean compromising exactly the thing that makes the show work. Season 1 already ran 28 episodes across two cours, so 10 is a real drop, not a rounding difference.
It does mean you're getting cut off right as it's getting good, no way around that, that's just where the story naturally stopped for this batch. But between the two reasons, it reads much more like "we'd rather give you a shorter season that doesn't compromise" than "we're rushing this out." More seasons are basically guaranteed eventually, just don't expect them back-to-back.
