There seems to be an issue with linked cards as users with external guest accounts can't navigate to the linked cards. I have attached a screenshot which shows the issue, but basically they see the box for the linked card and when they click the three dots on the corner, they see the title and remove button, but the line with the "Go to card" button is missing.
And they definitely have access to the linked card and the board that the card is in, the board is in the same collection and they can go to it by manually looking it up and opening it.