I would bet it's from AI upscaling. The dark edges around high contrast borders, plus the pronounced and slightly off-colour antialised edges (especially visible on the right side of the J) remind me of upscaling models.
Qalculate <https://qalculate.github.io/> is my favourite REPL-like calculator, although it unfortunately lacks an iOS app. It feels similar to using an HP 48-series calculator.
Numbat <https://numbat.dev/> is similar, but more CLI/REPL-focused, and with more of an emphasis on being a programming language.
I use KOReader's RSS reader, I haven't run into any issues yet. It supports RSS and Atom, supports pulling the full article text if it's in the feed, supports images, and puts each feed entry into its own epub.
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