noun which may take the genitive case particle 'no'
unissued; unpublished; not yet issued
Describes something that has not yet been officially released or published, such as stocks, tickets, books, or certificates.
未発行の株券はまだ存在しない。
Unissued stock certificates do not yet exist.
その本は未発行のままだ。
The book remains unpublished.
未公開 means 'not yet made public' and can refer to information, films, or events, whereas 未発行 focuses on items that have not been issued or released.
Compound of 未 (mi, 'not yet') and 発行 (hakkō, 'issue, publish'). The reading みはっこう follows standard rendaku voicing of 発行.