Charlie

Is it failing on the earth continuity test or the insulation resistance test?

If it is failing earth continuity, there is a chance that the external metal casing of the extractor is not directly bonded to earth, or is isolated from the earthed parts internally.

Do you have the model number? It would be worth checking the manufacturer’s instructions to confirm whether it is a Class I or Class II appliance. Even if the outside is metal, the motor and internal components may be double insulated.

Failing that, if you can take the ducting cover part of it off easily you may be able to see where the earth bond is/should be.