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I believe that your issue has to do with the PHP version you are using. Upgrade to PHP 7.1 or 7.2 to resolve this issue.

The anti-spam bad words feature works through regular expressions. Regular Expressions need to be compiled and run against some text, returning matches (or signalling the complete lack thereof). All programming languages which support them, such as PHP, use a third party library to handle that bit. In the case og PHP, that's PCRE (Perl Compatible Regular Expressions).

There are two major versions of PCRE. The first version was originally released in 1997, that's over 20 years ago. The new version was released in 2015. The old version does not support some extended Unicode characters, like some of the Chinese glyphs your pasted, whereas the new one does.

PHP 5.4 is truly ancient. It was first released in 2012 and its very last version came out in September 3rd, 2015. As such, predating PCRE2 for more than two years, it uses the original PCRE version which does NOT properly support extended Unicode. PCRE2 was only added to PHP in PHP 7.0.

Which brings us back to your issue. We cannot reproduce it with any version of PHP 7. This tells us that the problem is not a bug in our code but rather the use of the very old PCRE in PHP 5.x versions. PHP 7 versions using PCRE2 work fine. Therefore, you just need to update the PHP version running on your site. There's nothing else anyone can do about it.

Ugh, you're stuck between a rock and a hard place. The only thing you could possibly do at this point is ask your host for assistance. They should have a server-side security solution, e.g. mod_security2 for Apache, which can be set up with rules to block suspicious requests. Per Google Translate some of the Chinese text is very clearly something you won't be ever legitimately using on your site (the word for "diploma" come to mind) so they should be able to create a rule to block requests containing it.