How do I solve this very odd boolean problem?

If you're saying that it switches a bool to true that doesn't exist in the function then it must be elsewhere in the code, or if that was an example and you mean it is switching a different one in the function to true then it could be any number of reasons. We can't see when the function is triggered.

To make it foolproof I would bring the overlap family condition out of the function, and with that call the function, so it is like player overlaps costume, then call costume pick up function, this is more standard. You can send the family UID through the function as a parameter so you know which one was picked, in fact you would have to as things inside functions do not relate to instances picked by conditions outside of them, then use this family instance picked by UID to check its variable and such.

Thank you lionz for replying, I might not have explained myself too clearly.

The following screenshot shows all the references to costume_unlocked_sunshine in the entire series of event sheets

So, as you can see there is no switching of the boolean at all (we just check it in a different area)

We have made the adjustments to the function, so now the triggering of it looks like this

Which runs the following function

We interact with the Fire Costume trigger, (as you can see in the first screenshot) and it all runs but still the Costume_Unlocked_Sunshine boolean is switched and not the Costume_Unlocked_Fire one.

So very strange!

Strange and impossible lol. You'll have to share the project file to work it out. Are you sure it's set to true, what makes you think so?

you're not picking any costume objects on overlap