Cipher using multialphabetic single-loop ordinary substitution

Question

There is a program like this that encrypts a sentence in Russian, replacing the characters of the alphabet with the characters of three other alphabets. However, when deciphering a sentence, there are problems: deciphering in some places can go wrong(mainly if there are other characters (not letters(spaces and punctuation marks))), and some characters are simply "eaten".
https://code.sololearn.com/c0A3A10a132a/?ref=app