For a project with multiple target languages, You cannot select TMs for more than one language at once when we don’t have TM with multiple languages.

For example, I have a project en>es,fr and I have 2 separate TMs for this client en>es and en>fr. I’m not able to add them both in one go, but instead I have to add them one by one. That issue doesn’t exist if the client has one TM en>es,fr in that case I’m able to add that in one go.