Had this previously happen with a profile post that I thought might be intended as a personal attack against someone, so used "soft delete" to remove from public view while I looked into it. After I ascertained that it was harmless, I also found that I could not reinstate the profile post.
And I've just had a staff member accidentally click on the wrong button (perhaps "spam") on the user pop-up - which has soft-deleted all posts by that user.
Although I can select to "show" such posts, and read them, they are appended with the message "This message has been removed from public view" and any attempt to approve the posts will not work.
I'm reporting this as a bug, because it doesn't make sense that a xenforo admin has the power to soft delete, but has no recourse to reinstate.