A grenade chucked into the back of the Van is a considerable inconvenience to an Astral projecting mage. Not so much a Spirit on the Astral plane.
There is a distinct mechanical difference between an Astral Mage and an Astral Spirit. An Astral projecting mage is mechanically similar to a VR Matrix character. The character's perception is shifted to a different plane and often a different "place". But the Meat sack is still physically somewhere and needs to be tended to in some way. Stuffed in a Troll's back pack or dumped in the back seat or whatever.
An Astral Mage still has Foci, and links to those Foci, and a Signature that can be tracked back to the Physical body. An Astral Spirit has none of that. Foci would be left behind on the physical plane and arguably stop working. A Spirit's signature isn't going to lead you back to the Spirits physical form if the Spirit is on the Astral Plane.
Dual Nature, Astral Projecting, and Astral Spirit are three mechanically different things with different rules.
An Astral projected mage sustaining a spell runs the risk of someone running down the signature to a helpless body.
If you need in game metaphysics, the body matters. The body, being on the physical plane, is able to sustain the spell. The Astral Spirit, without any physical presence, has no direct link to the physical plane and can't manipulate any physical energies on that plane.
RAW arguments: A spirit needs to be on the physical plane to use a P power. Someone gaining a benefit from that power is, by definition, using that power. If the Spirit is not on the physical plane, a P Power can't be sustained. Or if you'd rather, the P Power gives no benefit. If you want to say it's still sustained and will "re-activate" when / if the Spirit manifests on the Physical plane again, sure, whatever.
RAW: A mage can sustain a spell, and they take a penalty for doing so. There are no other restrictions on sustaining. If a mage was meant to stop sustaining a spell when they went Astral, either the Sustaining rules should state the Astral Projecting exception. Or, the Astral Projection rules should state "Sustained spells are ended or suspended."
RAW its sometimes okay to assume the lack of an exception is permission to do something. The rules don't clearly state that you can sustain a spell while driving, for example. I don't think anyone is going to argue that sustained spells quit when a mage is driving.
And quick strawman, would you argue that Quickened spells are stopped when a Mage Astral Projects?