I had the same issue a couple weeks ago or so.
It was a setting in Camera Raw Preferences within Photoshop.
You can set it to automatically open Tiffs and or Jpegs in ACR.
But, I don't want the exported C1 TIFF files to open in ACR. I want them to open in PS.
My workaround:
It's so fast and easy to view files, then open them in ACR to get a closer look. Inevitably, I'll end up making a few gross adjustments to test the integrity of a file--dynamic range, focus, noise, etc. Then, afterwards, I made the mistake of importing those same file to C1.
I discovered that when I imported a file to C1 first viewed in Bridge/ACR, C1 processes and exports it as a .tif--the problem being it'll only open in ACR (despite the .tif suffix). This doesn't happen when importing a file that wasn't initially viewed in ACR. C1 likes virgin RAW files.