It seems the problem is only graphical.

1. It's 4 blue players, 6 red players, I queue as "waiting to join red team"
2. The button turns grey.
3. It still correctly joins me into red team when a slot becomes available.

The issue itself, I have seen happening after another player becomes spectator.
Not sure if it starts happening because player limit is exceeded by patreon players...

@bunniewormy ping, I missed that issue.

In SpectatorQueue.as:

void onPlayerChangedTeam(CRules@ this, CPlayer@ player, u8 oldteam, u8 newteam)
{
	if (newteam == this.getSpectatorTeamNum())
	{
		SetupQueueGUI(this);
	}

	if (oldteam != this.getSpectatorTeamNum() && player.isMyPlayer()) 
	{
		hide = true;
		return;
	}

	if (this.get_bool(player.getUsername() + "_playsound"))
	{
		if (isClient() && player.isMyPlayer())
		{
			Sound::Play("AchievementUnlocked.ogg"); // TODO: different, distinct sound
		}

		this.set_bool(player.getUsername() + "_playsound", false);
	}

	client_selected = -99;
	RemoveFromQueue(player);
}

I think the line client_selected = -99; is the cause of the problem, making your button grey when someone before you switched teams. Maybe the issue is fixed by moving the line into the if (oldteam != this.getSpectatorTeamNum() && player.isMyPlayer()) code block. But I'm currently unable to test it.