Disjointed match on one-way synonym - how to fix this?

I am using Synonyms to match decimals to fractions for some of our product descriptions. So for example, we write mixed fractions as 1-1/2". If I create a synonym for 1.5 → 1-1/2:

then in my search, when I search for 1.5, I get some of what I want:

But then also a lot of loose matches that don’t really make sense:

You can see that the 1 from the front of 1-1/2, matches to a 1 further in the string.

I know that these is an exact matches setting. Can I make it play nice with synonyms to get rid of this bad behavior?