Following up on this post I created - Need help modifying the saved value of a custom taxonomy - I think the problem could be with the PayPal add-on.
To recap: My form allows users to pay to create a 'ticket' custom post type from the front end. The problem was that the terms were not being properly saved on the resulting CPT.
I tested the same form but without PayPal integration and it worked fine (Screenshot: http://skit.ch/ndf2). Tickets #107 and #108 were created by user John Doe without PayPal integration. Ticket #109 was created by John Doe as well but the Status, Priority and Type taxonomies were not saved. The author (Submitted by) was saved as the default post author, not John Doe who submitted the form.
I also tested this against regular posts to rule out that the bug exists with Brad Vincent's GF + Custom Post Types plugin (screenshot: http://skit.ch/ndf3). Here we see that the categories and tags are applied and not left blank but post author is definitely wrong (should be John Doe).
Anyway, I realized that leaving the "Create post only when payment is received" option unchecked will not cause the bug so that's what I'm doing at the moment.
I'm just reporting this here so that you are aware of the problem. Hopefully you guys are able to fix this, and save someone else from making lots of payments to PayPal while trying to figure this out!