Ran into an odd one today.
If you have a Google Group where the 'Who can see group' setting is set to 'Group members', and you add the group to a space, along with other users (or other groups), if someone tries to mention (@) a user in that space they will get a message asking if they want to add the user to the space (even if they are already a member through their group membership). Depending on whether or not the user is allowed to add members to the space, the mentioned user will be added to the space explicitly, outside the group, or the user that sent the message will get an error - and the mentioned user will have a strikethrough text formatting.
This likely happens because the user that mentions the other user isn't allowed to see if the user they're mentioning is a member of the group or not, but trying to add the user to the space probably shouldn't happen?
Did you ever find a solution to this? I'm running into this as well.
No solution yet - but most of our Spaces use groups for managing membership, so I haven't heard any further complaints of this coming up from users since the first occurrence. Kind of an edge case for us, and probably not even on anyone at Google's radar (I don't know if anyone on the product management teams ever read these forums, they seem kind of DOA to be honest.)