Thank you for contacting PayPal Merchant Technical Services, I apologize for the delay in responding to your email.
We are aware of this issue with our IPN Simulator and our development team is working on a fix. Until this issue is fixed, I'd suggest running your IPN tests by making payments in Sandbox as we do send IPN requests with every payment.
If you have a similar issue with your payment tests, please let us know and we can take a look.
My apologies for the inconvenience caused.