Not sure why this isn't working as I use it another Flow and it works just fine.
The API I'm working with, when I PUT goes to a URL like this: /custom_field_value/{id}.
Based on the ID, it dictates the data type that it will take. If it is string, the BODY needs to be structured as the following:

If it is in array, it needs to be structured like so:

My array ones are not working, and are receiving double quotes, while others are working perfectly fine. Here are the relevant parts of the Flow since it is quite lengthy and can't be posted here since the image would be too big.
1. Declare the empty array these values will go into:

2. If the ID is one that is a multi-choice field (an array), first split the string (Dynamics CRM stores them as a string with commas separating the values, but the API wants an array of strings or integers), then iterate through that created array and append to the empty multiChoicefield array. In this case, I'm converting them to integers, but it takes either (tested via Postman).

3. Then send the parameters to the API. (The if() is just checking whether it should send a string or an array).

This is what happens when it fails. Flows doesn't make it easy to see what exactly was sent in the BODY of a request, which is pretty annoying.
4. Failure despite everything looking correct. Fails because "Value is invalid because one or more choice is not on this custom field" which is not true.

So see exactly what is going on, I have to go into the custom connector and see what is happening.
5. Copy and paste what is being shown for the "id" and "value" into the Test portion of the custom connector:

6. Of course it fails, so check out the Raw Body and... yep, it is adding double quotes around the array. Yes, the connector has this key as type of string, but it is like that as well in other Flows where this works perfectly.

7. So what should it look like if I fix the Raw Body here:

8. And it makes it look like this:

Submitting this works just... but if you just type in exactly what you see here, you get the following in the Raw Body:

This is incorrect and ignores the second value as you can see here:

So I am stumped. Especially since I do this in other Flows and works just fine. Even though the custom connector key is of data type string, I can pass in arrays without issue in other Flows I have... just this one is causing a very iritating issue.