The "prop" column is set when a value is set to the prop in the adobe call. E.g. c.prop1=red. "red" would show up in the prop column and also the "post_prop" column.
The "post_prop" column is set with the value after the value is modified on the Adobe side by a processing rule, a vista rule, or some other transformation after Adobe receives the value. The value in the "post_prop" is what you would see in Workspace. If there was no post-receipt processing, the value would be "red".
The only scenario I can think of is that there is a processing rule or vista rule that blanks the value that was sent.
You could check Workspace. If you sent red and in Workspace there is nothing, then that would support the above scenario. On the other hand, if you see red in Workspace then Adobe has received and processed the call correctly.
You would then check the raw data feed to see if red was there. If it is in Workspace, it is almost certainly in the raw data feed and the issue is happening after you have received the data feed.