Thank you. I accidently discovered the same thing on my 2nd time through the game. I felt like an idiot, to say the least. I was trying to get a Scavenger Hunt upgrade from the wrong Ford Cruller!
One thing everyone should know, though, is that you do NOT get an upgrade for your completed Scavnger Hunt when you pass the Point of No Return. I went up to the Fortune Teller's booth at the beginning of the end level and the dude didn't give me anything for the completed Scavenger Hunt.
I was still able to complete the game but I didn't get up to rank 100 or even 95. I think I ended up at 94 (I kept missing figments, especially the ones that drift by, like in the Basic Braining rotating log path).