That's the way it works. ID has no way of knowing what is a fraction and what isn't, and the feature is intended to be used by selecting the specific text.
Only use that as a character style. Only apply it to what you want/select.
Make a GREP Style in the Paragraph style, where you train it to look for \d+/\d+ and if found, apply a character style (I like to call Frac-a-matic, where you have turned on that one OpenType feature).
Other GREP commands could be added, such as: (?<=\s)\d+/\d+(?=\s) to further limit it from bothering other number strings (in this case by requiring a spacebar space on either side).
GREP Styles within Paragraph Styles are amazing!
Hmm, ok. I didn't realize that.
I had taken InDesign a little bit smarter than that but the GREP style option might be a good solution. Thanks a lot for the clear answers!
[As far as I can see the GREP style works excellent in my text! Thanks a lot again!]