Look closely at what you are doing with JiraIssueList - you get its content and split it as though it is a string. Is it really a text field with a list of issue keys in it? Do you actually trust your humans to get the format right every time?
Assuming it is a valid list like that, then think of the text in it - it's an issue key. So when you cast it to a mutableissue, the result is utter nonsense - an issue is not a string of characters.
Your log will tell you that line is nonsense whenever it executes, and because it is, the script will stop at that point.
Step back and look at the actual type of field 10083 is - is it really a string? Or is it a select list or issue picker? Once you know what type of data is really in it, you can parse it and use that data to find the issue it is pointing to.
Yes, that's what I said - you can't just cast an arbitrary string to an issue.
Your code needs to work out what issue the string is pointing to (if that is a valid pointer - you really do need to check that the string is actually valid, as a human could be putting anything into it)
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