WordPress rewrite Solution

I have been plagued by a problem these days. Because my WordPress is stored in the root directory of the website, some sub-directories I have created run some other applications, I found these applications are WordPress. the rewriterule of the htaccess file is affected. the htaccess file allows the rewriterule of the subdirectory to work normally, but it is found that the application that calls the subdirectory always returns the 404 status, but the content is normal.
In fact, if you create a sub-directory to put the discuz Forum, the Forum's rewriterule will also be disturbed. This problem is really confusing, especially when the 404 status is returned, all files will not be indexed by search engines.
After some debugging and modification, I found a strange way to solve this problem, that is, add the following line to the PHP file in the subdirectory. Code:
Header ("State: 200 OK ");
After that, I used some HTTP status testing tools to test the Directory and file. Instead of returning the 404 status, I returned the 200 status. Then I will observe the indexing of this subdirectory in the search engine, it is estimated that it will return to normal. Some of the features of Wordpress are really strange. I don't know why.