The question is falsely premised on several counts:
If we look at Section (2) it provides that
An application for admission for assessment may be made in respect of a patient on the grounds that—
(a) he is suffering from mental disorder of a nature or degree which warrants the detention of the patient in a hospital for assessment (or for assessment followed by medical treatment) for at least a limited period; and
(b) he ought to be so detained in the interests of his own health or safety or with a view to the protection of other persons.
Please NOTE - the decision is not one of capacity. That is not at issue in the decision by the two doctors and the AMHP. At issue is simply does he have a mental disorder the nature or degree of which warrants detention for his own or others health or safety.
Equally the MH tribunal (following an application under S70 or S71) is not required nor has the power nor indeed would it likely be able to find whether that admission was wrongly made in the first place - that is not within the power of the tribunal.
Instead under Section 72 the Tribunal looks at the CURRENT situation - not the situation at the time of detention (albeit that it might have bearing.)
The language states “he is then” - meaning at the time the Tribunal decides.
Here is the language in the Act
(1)Where application is made to the appropriate tribunal by or in respect of a patient who is liable to be detained under this Act or is a community patient, the tribunal may in any case direct that the patient be discharged, and
(a) the tribunal shall direct the discharge of a patient liable to be detained under section 2 above if it is not satisfied—
(i) that he is then suffering from mental disorder or from mental disorder of a nature or degree which warrants his detention in a hospital for assessment (or for assessment followed by medical treatment) for at least a limited period; or
(ii) that his detention as aforesaid is justified in the interests of his own health or safety or with a view to the protection of other persons;
There may be any number of reasons why the tribunal may direct the discharge OR not be satisfied that i or ii are the case: for example - the current crisis has passed, the patient has been medicated and is stable, the clinical team are not opposing discharge and have not put forward evidence, the clinical team may not be intending to pursue a Section 3 continuation of the detention or even perhaps that the clinicians lack time to provide evidence. This does not mean or imply that the admission was unlawful (or “legally wrong,” as the question phrases it,) in the first place.