I too am seeing this. It started a couple of days ago, and I too am only seeing it on certain transactions. Here's an example sent to my IPN handler; please note the entry highlighted below in bold: IPN Validation failed for ticket: 12576, reason: NO SUCH PRODUCT.
mc_gross=29.95
settle_amount=21.87
protection_eligibility=Eligible
address_status=confirmed
payer_id=RSNDSAMQZ9V3E
address_street=<redacted>
payment_date=00:36:46 Apr 04, 2017 PDT
payment_status=Completed
charset=UTF-8
address_zip=<redacted>
first_name=<redacted>
mc_fee=1.86
address_country_code=AU
exchange_rate=0.778676
address_name=xxxxxxxxxx
notify_version=3.8
settle_currency=GBP
custom=
payer_status=verified
business=<redacted>
address_country=Australia
address_city=SYDNEY
quantity=1
verify_sign=AYm-UvofG9njKKDeEI8KzfvIh05IAOmferjFfopEa6-R3s16mJ2-.mIV
payer_email=<redacted>
txn_id=7WA77895A38926730
payment_type=instant
last_name=<redacted>
address_state=Nsw
receiver_email=<redacted>
payment_fee=1.86
receiver_id=NDG6E3UMYY7LS
txn_type=web_accept
item_name=VinylStudio+for+Macintosh
mc_currency=USD
item_number=12576
residence_country=AU
transaction_subject=
payment_gross=29.95
ipn_track_id=9c6b1f2089291
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