Just to clarify for the poster, this is not a bug and is as it should be
However, the Help File for 'Detect Hidden' states
...which is incorrect. Detect Hidden only allows detection of hidden objects and concealed or 'secret' passages/doorsDetect Hidden
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Divination
Range: Personal
Target: Self
Syntax: cast 'detect hidden'
This spell enables the caster to detect hidden passages and creatures.
It does not allow for the detection of invisible objects.
Hiding creatures can only be detected with 'detect invisible' or 'true sight'
The Help File has now been corrected, many thanks to the poster for bringing this to our attention