Hi @String ,
I have tested it on my side, these are screenshots of flow configuration to refer:
Firstly, I initial a variable to store the object of TestObject.
Secondly, initial an Array to store the category of TestObject, such as category1, category2, etc.

Thirdly, add apply to each to get each value of Category Array. Then determine the value of the category from TestObject whether equal to true.
-If yes, output the corresponding value of Category Array.

This is the screenshot of run successfully:

Please have a try, I hope it will help you.
Best Regards,