I have a weird problem and am hoping someone can point me in the right direction...

ZC 1.5.6, OPC 2.1.3, php 5.5.38

Using PayPal Express and Guest Checkout sometimes comes up with a blank shipping address. Keyword is sometimes and I can't figure out why - the only common factors are PayPal Express and Guest Checkout. It does not happen all the time, so we do have guest orders using PayPal Express and it's all good.
We tried disabling PayPal Guest account (in PayPal account settings), and have tried disabling the PayPal shortcut button on the login page - made no difference.

Attachment 18620

One thing I did notice (although possibly just a coincidence) is that there are 0 orders using PPE with Guest Checkout and different billing/shipping addresses. We do have PPE and different addresses, we do have PayPal Pro and different addresses, we do have PayPal Pro in Guest Checkout with different addresses, but not one PPE in Guest Checkout with different Shipping and Billing addresses. Maybe just a coincidence?

Any ideas? Where do I start? I have both PPE and OPC debug on, but didn't notice anything relevant. TIA