Let me join in this thread, mpeterson.
There is a reported issue about IIF files not importing into QuickBooks 2019. Our engineers are already investigating it and they're working diligently to get a fix as soon as possible.
I suggest giving our QuickBooks Technical Support a call so they can add you to the list of affected users. Here's how:
Rest assured, our team is doing their best to get a resolution for you. If you have other concerns, you can always visit us anytime.