As mentioned, this is because mIRC is now a Unicode application and will only send and receive UTF-8 encoded text. The purpose is to, ideally, make IRC as a whole non-reliant on codepages, so that everyone will be able to view all characters in all languages. In older versions of mIRC, every user had to manually configure each of their channel/query windows with a specific font and codepage to display one particular language, and this only worked for that language, so if someone came onto that channel and spoke a different language, it would appear as garbage.
That said, the above IRC server is sending a numeric that indicates your current codepage:
<- :irc.tomsk.net 222 arnie CP1251 :is now your codepage
Unfortunately it is sending it after numeric 003 and the MOTD, which both use codepage-specific text. In order for that numeric to be useful, it would need to be sent before any codepage-specific text is used, so that mIRC can convert text from that codepage to Unicode. However, even if mIRC did convert codepage-specific text, there could still be issues, such as overlap/conflict with other encodings. This is another reason why mIRC is trying to become a UTF-8-only client.
I realize this is going to be an issue - there are many networks/servers/channels that use codepage-specific encodings. I am hoping that the benefit of UTF-8, which has been supported by mIRC since 2006, will be enough of an incentive to convince many users to move away from codepages.