I have a flow where I am looping through related detail records and adding their names to a flow variable "Product Name" - data type text, then adding that variable to variable collection "AllProductNames" - data type text.
Not an actual solution to your goal, but the documentation explains why you get the behaviour you see:
true if: An item in the collection that’s selected for Resource contains the exact same value as Value
I.e. it does not find a partial match, only an exact match.