Some org syntaxes use hi def link to markdown… (e.g. hi def link orgBold markdownBold), so if markdown syntax file has not yet been loaded, they are not highlighted.

For this simple t.org file:

* h1

formatting: *bold*, /italic/

, when opening it with

vim t.org

, * h1 is highlighted, but not *bold* and /italic/.

A workaround is to use :setl ft=markdown ft=org.

A fix would be adding runtime! syntax/markdown.vim (before let b:current_syntax = 'org') in syntax/org.vim.