Your problem could be related to the use of Korean characters indeed.
If possible it would be interesting to make a simple test with similar data but using only ascii characters to check the results.
Also given the characters are broken it is very important to ensure you have the parameter -Dfile.encoding=UTF-8 in the Tomcat JVM parameters.
See the Installation guide, search for UTF-8 and you will find the references depending on your OS.
Hope this helps
Could you clarify what do you mean by "And I confirmed that it was ###." ?
- which version of ThingWorx are you using ?
- on which OS is it running
- on Which OS is the client
- have you tried the same thing with only ASCII characters ?
Note that your issues is probably related to https://support.ptc.com/appserver/cs/view/solution.jsp?n=CS255971 . This article does give some workaround to make it work in Excel, so it is probably worth reading through the different workarounds and try the one that work best for you.