This person also is experiencing the same issue: https://www.games2gether.com/amplitude-studios/humankind/forums/215-bug-reports/threads/50703-pc-steam-v1-0-13-2723-s-10-embassy-menu-does-not-show-in-multiplayer

Sorry forgot to include some information

• Title: [PC/Gamepass, v1.0.17.3494-S10 (build: 261337] 
• Steps to Reproduce: Don't know how you would test this on your own but we played 3 games and all 3 games they could not access the embassy menu unless prompted. 
• Not quite sure how to find my cloud saves.

Same here, the player which did not own the DLC could not see the embassy tab. It didn't appear at all (not greyed out like when you don't have an embassy). He was able to access it if via my embassy I sent him proposal but not tab appeared to be selected.

Same here.  I hope this is a known issue that's being worked on, because it made the diplomatic cultures worthless for any of the other players in the lobby besides the host.  These players could find leverage intel on the map and pick it up; they just couldn't see the menu where they'd actually use it unless they had a notification there pending.

same

I am having this same issue but as the person it is happening too. I have a friend who plays this all the time and owns every DLC. So I hop in to play with him every couple days, but halfway through our game, he asked why I hadnt sent any diplomatic stuff. I didn't even notice that there was a new mechanic because the tab never showed up, and him or the bots never sent any diplomatic stuff.

To recreate the issue: Get to the point where you can build the embassy in game, then attempt to open the embassy tab when interacting with another empire. If you do not own the DLC, it will not show up.

Thank you and I hope this is resolved quickly! 

Thank you very much for your report!

An issue was filed for this bug.

Best regards :)