I haven't read any Dostoyevsky since high school, and don't remember it at all, but I'd imagine it has to do with nicknames.
A non-Russian speaker is going to be confused when the same character is referred to as both Alexander and Sasha, for example, and will think they're different people.
I think it's the translators' fault. I think they could've added some footnotes like *Sasha - diminutive of Alexander.
This could be an LLM based e-reader feature, replace names with non-native intelligible translations.
I've never understood this particular issue. My mother had it too with Russian literature. Many Russian books have a "cast list" at the beginning to get round this. I don't find it any stranger than William being called Will, Wullie, Bill or Billy; or Robert turning into Rob, Robbie, Rab, Bert, Bob or Bobby; or Elizabeth being turned into Liz, Lizzy, Beth, Betty, Liza, Lilbet etc. I found most Russian diminutives are formulaic so I picked them up fast.
Sasha may also refer to Alexandra, which is a feminine first name. What's more, there's like a ton of diminutive short names for these -- my first ever instagram link on HN, but: https://www.instagram.com/reel/DTUeNn7iAit/
(FWIW, it lists: Sasha, Sashka, Sashulya, Sashenka, Sanya, Sanechka, Sancho, San, Shurik, Sashunya, Sanyusha, Sanyok. I myself have heard native Russians use Sash - should be written as Сашь -, and e.g. Mish - Мишь -, which is a similar "lazy" conversational short form for Misha/Mikhail.
I've learned some Russian, and once you start sensing the endless magic they can do with verb prefixes and sufixes, you realize what a versatile language this is. Somewhat the same counts for first names, I guess.)