Well I suppose it's up to interpretation. The card says "Lasts until building destroyed" . Since it applies to newly spawned units (as it is expected), it should re-apply to disenchanted units too (at least I think that's the correct mechanism).
The effect of Disenchant (and Sobering, which is caused by the Lost Spellbreaker) on the other hand state that the unit is insensitive to major buffs. So it doesn't even state that it should remove the buffs, they should be still on the unit, but have no effect.
Again, this is a question of interpretation at this point (or the descriptions can be misleading, they may not reflect the original implementation design). To me it is clearly a bug, and after inspecting the description of the Sobering and Disenchant effect, maybe it isn't even a bug of the buildings, but a bug of the effect, removing every buffs.