How does that play with BSD license file which presumably has no such restrictions: https://github.com/facebook/zstd/blob/dev/LICENSE
1) BSD license covers copyright, and that patent file covers patent grants. Totally separate things. If you sue Facebook, you retain the BSD license and they can't sue you back for copyright infringement, but they can then come after you asserting you infringed their compression patents and demand royalties.
2) Because PATENTS file says "Additional Grant of Patent Rights Version 2" it could mean an extension of the BSD license. That is the termination would trigger the revocation of copyright license as well not just patent protection.
Does termination of the additional patent grant in the Facebook BSD+Patents license cause the copyright license to also terminate?
No.
I don't know what are the consequences of the Facebook patent clause, but I know the BSD license doesn't contradict it.