I have the same issue and changed my script to compensate, but overnight I am now seeing that the 'txn_type' field has changed from 'web_accept' to 'cart' on the orders that have this issue. I have never used cart so should not be seeing a 'cart' response. For some reason PayPal are sending cart responses for none cart orders. I am also still seeing the normal orders come in (web_accept) for some customers, it seems to be random as to which arrives, its around 50/50 at the moment. I contacted PayPal a couple of days ago but have had no reply yet. I noticed that other posts are appearing with the same issue. If anyone gets a response from PayPal on this issue then please let everyone else know.
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