tablefield.module doesn't seem to support multibyte characters

I've noticed that when importing CSV files with characters like "ñ" and sometimes with leading spaces, this warning is generated for every instance of the character:

Warning: htmlspecialchars(): Invalid multibyte sequence in argument in check_plain() (line 1521 of C:\inetpub\wwwroot\ewbcci\includes\bootstrap.inc).

After digging in DO I found out that this has been happening in other modules, like as mentioned in this post, this post and especially in this post because I'm also using IIS in my project. There is a temporary solution here but I don't think editing bootstrap.inc is wise.

Has anyone encountered and have tried to resolve this issue by any chance?