Disabled multiplayer in versions 1.16.4 and 1.16.5

[WinnLov]

As of snapshot 20w21a, two flags have been added, --disableChat and --disableMultiplayer. This allows multiplayer to work up to 1.16.3 and from 1.17 onwards, but in 1.16.4 and 1.16.5 multiplayer is disabled. 
The question of how to disable multiplayer simply by adding a flag to the startup arguments has already been raised, but I'm wondering if there is a way to disable or ignore the flag at startup so that multiplayer works on 1.16.4 and 1.16.5.
I've dug through a bunch of sources but haven't found a way to solve this problem, I realise that work on the project is now frozen, but maybe someone has been able to solve it or knows how to do it.
Thanks in advance :d

[Tartilla-TAIGO]

Hi. This is an authlib bug. It is observed only on these versions in offline mode. Many solved this problem by simply decompiling authlib and fixing this error.

[Tartilla-TAIGO]

You can change the authlib by path libraries\com\mojang\authlib\2.1.28 to this file before launching minecraft
authlib-2.1.28-workaround.zip

[WinnLov]

Thanks, it looks like it really is an issue with authlib. Thanks again!

[Tartilla-TAIGO]

You may need to use authlib-injector using official Mojang servers. I have not tested with the official Mojang server, but with my own authentication server, the online game button is active.
https://github.com/yushijinhun/authlib-injector

[AlphaBs]

easy solution: add extra JVM arguments

var process = await launcher.InstallAndBuildProcessAsync("1.16.5", new MLaunchOption
{
    Session = MSession.CreateOfflineSession("username"),
    ExtraJvmArguments = 
    [
        MArgument.FromCommandLine("-Dminecraft.api.env=custom -Dminecraft.api.auth.host=https://invalid.invalid -Dminecraft.api.account.host=https://invalid.invalid -Dminecraft.api.session.host=https://invalid.invalid -Dminecraft.api.services.host=https://invalid.invalid"),
    ]
});t