Guild chat bug?
Are you representing the guild?
When you accepts a guild invite you then has to choose to represent the guild, and while you may join multiple guilds you can only represent one at a time and it is the represented guild where you see chat from and similar.
yes I am representing the guild…
Sounds like you may have accidentally turned off guild chat in your chat box. To the left of the text line there is a little arrow, press that, this brings up the chat window. At the top of the window there is a tab called ‘Main’, next to this there is a little down arrow, press it and see if Guild is selected.
Hope this helps