Just a wild guess, but does the file you're trying to open have a side car .xmp file for it ?
Could PS be looking for an xmp and if it finds one assumes you want it opened in ACR ?
No XMP files (not really necessary) the edits from ACR are seen within the document in File Info. That is what I assume ACR 'see's when the preferences are set to use it and it opens there. What was interesting was saving out a copy didn't fix the issue because, well the metadata gets copied.
Bet if I used Save for Web and stripped it, it would open normally