Without dev-python/python-markdown-math installed, retext does not display a preview, but instead shows a message saying that the markdown package is not installed. In my case, dev-python/markdown-3.7 was installed, but dev-python/python-markdown-math was not. Installing dev-python/python-markdown-math allows the retext preview to work. I thus think that the ebuild for retext (currently /usr/portage/app-editors/retext/retext-8.0.2.ebuild) needs an RDEPEND for dev-python/python-markdown-math It is possible that dev-python/markdown should be amended instead - but I don't know enough about Gentoo's Python module policy to suggest that.
Same issue, after installing dev-python/python-markdown-math manually, retext preview starts to work again.
I have observed the same behavior.