With a fresh install of salt, I was seeing errors matching the bug report here:
It has to do with the byte compliling python modules that FreeBSD does in a similar fashion to what Gentoo does when installing.
Pull request to fix it is here:
applying the following patch as a user-patch fixed the issue:
supposedly this is to be in the 2018.3.3 release, though I don't think it's been merged yet.
Tested salt's 2018.3.3 git branch. Solves the issue. Bumped ebuild, adjusted SRC_URI and removed second patch from PATCHES. Just in case someone is interested.