Might just be on my end, but I do find it weird that using the header directly works.
Edition: Windows 10 Education
OS build: 17134.648
Hmm. If it isn't working for you, it will likely not work for others as well.
If you use SmartSniff to monitor http packets, can you see if mIRC is sending the Basic Authorization header? This will only be visible with http and not https. For https, you would need to install something like Fiddler (which installs a root certificate to enable https decryption).
Also, can you let me know the type of server you are logging into? Is it linux/windows/apache/cpanel/amazon/etc.? I might be able to set up a similar service to test it out.