hmmm, I can see this being an issue. In my test, the purchase receipt notes gets triggered when you tab out of the product field on the purchase receipt function, however, in the case when you receive against the PO, the product field if filled in automatically without the need to tab through of the Product field. I would suggest you open a ticket with Sage Support for this. 

Hi Allison!  Have you been able to get this to work for you?  I had the same results as you and was wondering if there was a solution.  Thanks!

I haven't.  I also didn't open a ticket as this wasn't the highest priority.  

Thanks.  I'll share the solution if we find it.