Technical question on old bitcoin core USB wallets
I know someone that is struggling with an old bitcoin core wallet from pre-seed phrase days. They are unable to authorise any outgoing transactions when loading the wallet.dat file into a freshly set up copy of bitcoin core. This wallet has not been active since at least 2017 and the password has not been changed (I/they are certain of this as the wallet wasn't touched at all immediately following a transaction back then). This password worked previously but now they cannot get it to accept with modern bitcoin core.
I know that 99% of the time these things boil down to 'oh darn, turns out I did change the password after all', but here's the thing; the wallet is encrypted with a password in Russian Cyrillic. I read somewhere that Cyrillic letters can require some different kind of decryption, however my contact has successfully used this password many times up to 2017, at which point they stopped using the wallet and have been unsuccessful in accessing it since ~2020.
Are there any known issues with Cyrillic denoted passwords and modern versions of bitcoin core, and could this be remedied by downloading an older (circa 2016/17) version of core? They have incredibly slow internet so attempting this would take over a month and I don't want to waste their time.
I'd be happy to provide further details/try replicating any fixes with a blank wallet if anyone has suggestions.
Many thanks in advance.