You have to update the value of the File Content field in the "Upload File" task.
From: File Content
To: concat(uriComponentToString('%EF%BB%BF'),body('Get_file_content'))
Note:
- You can type the updated value by clicking the "Expression" tab.
- You can get the value of body('Get_file_content') by clicking the "Dynamic Content" tab and selecting the "File Content"

Explanation:
.csv file is a kind of text file. If you encode it in UTF-8 format, there are two ways to save it.
- Text only
- Byte Order Mark(BOM)+Text
BOM is 3 characters (EF BB BF) to mark the file is encoded as UTF-8.
When you generate .csv file in Power Automate, it does not include BOM.
So, when you open the .csv file in excel, special characters (i.e. Korean) will be broken.
The solution is to add 3 BOM characters in front of the .csv file content before saving it.
Let me know if this works. It is working fine using the "Create File" task. I haven't tested using the "Upload File" task.
Rommel Rueco