If you say Brahman is being unmanifest you are bringing Brahman as duality which is not advaita. How is advaita claiming unmanifest and still the non duel? If unmanifest, then opposite is there. Manifest is attribution therefore same same unmanifest. This cannot be non duel unless one explains with the achintya bhedabhed in which case the prospect of Lord and BRahman together becomes lively.
Other hand if Brahman is being compared like mango seed then that one is unmanifest mango tree. But seed no more once baby mango tree becomes. But if Lord is compared as the mango then the twig is called as mango, the leaf as mango, the fruit as mango, the wood as mango, the seed as mango - see alll is mango. The mango is the Supreme Attribute for the twig, the leaf, the woods, the seeds, the fruits. Without mango the mango seed is meaningless.
Therefore Brahman is in LOrd as seed is in mango. Seed is not separate from mango. It is being mango seed.
BG 14:27 Lord is saying Brahman is home in Lord. Non duel (advaita) state of Brahman is already reached by previous BG 14:26. Verse 27 is now decribing status of non duelty. Lord and Brhaman are non different but Brahman is in Lord. The relations between Lord and Brahman is subject for achintya bhed abhed.
Otherwise how you can say Brahman unmanifest (and therefore call in opposite prospect) and claim non duel?