Yes Epic, that works for that 1 server's certificate, but you'd get that for each of the servers connected to the network's round-robin address, and would probably get it again the next time that server changed their certificate.
The beta fixes the problem for me at least.
For those using older mIRC versions, I encountered this also with version 7.56 too. For versions prior to that, Libera.chat doesnt want to let it use an SSL connection because of using TLSv1.1. At another network which allows connection from 7.55 and earlier, it also had the same certificate warning there too.