Is someone who has been diagnosed with dimentia , are they still able to sign for medical release and other legal documents with out their POA involved?

Is someone who has been diagnosed with dimentia are they still legally able to sign legal documents for themselves without their POA signing?

Possibly yes depending on the severity of the condition. Having the POA doesn't take their rights away, that only allows another to act in their behalf.

A court would have to appoint someone to be a legal guardian of the person in order to make it so that the person with dementia could not sign such papers.

If papers are signed, it can be questioned as to if the person was in their sound mind when they signed.