Hello Matt,
I too have noticed this. Our site has had an SSL for years. This is the message I see under a donor who’s PayPal donation went through correctly according to PayPal but not Give:
System – October 15, 2016, 2:06 pm
Status changed from Pending to Failed. – Delete
System – October 15, 2016, 2:06 pm
Payment failed due to invalid purchase key in PayPal IPN. – Delete
The same donor has donated 23 times with PayPal, most recently (before the 15th) was October 8th.
Please let me know if there is any other information that will help out.
Thank you.