I don't think this thread will get a lot of activity - but anyways, no harm.

To not offer an upgrade is for a Stay Plus night is correct - that's normal and wouldn't surprise me.
However to not receive access to the lounge as a Plat is a bit of a slap to the face.

I stay at this property a lot and will be there again soon. Let me check if that is normal. I know they treat members very well there normally and don't shy away from granting even lower silver or classic members lounge access occasionally if the member appears to be frequent guests in future.

I normally redeem my Stay Plus in Australia at one of the Sofitels as I feel that gives best value for money. And I have not been denied access to the lounge on a voucher stay.