First off we all know how unions work they sometimes arent good for making doors. Mostly used for decoration. Ive come across this bug for my sound regions. I have tried to make 4 seperate parts around the building 
As shown here. I then unioned them all into one part. But when I test in game the music starts playing even when im not even touching the union!
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You probably are touching the Union, it just doesn’t visually look that way.
If you use this plugin:
you can click the tool and it’ll show you precisely where the Union is physically.
This may sound stupid but how do I use this plugin?
If it’s in your top toolbar in Studio, click on it. All your Unions/Meshes will turn to blocks of colour which show their actual shape. To turn it off just click it again.
Thank you for showing this but, even when im not inside it it still plays the song. But when I go above the union the music stops. https://gyazo.com/dd3627fd4c874773863b29ac0b0d4e7a So this really has to do with the union.
in this case, I would like you ask you to send your code. Most of the time people have problems it is because of the script itself