<Unreadable> for POST body length of 100000 bytes or above, also with blob/FormData

EDIT: eis was right, there was an interfering (Netbeans) debugger.

The client sends some data to a Spring Boot servlet:

        fetch("/command", {
            method: 'POST',
            cache: 'no-cache',
            headers: {
              'Content-Type': 'application/json'
            },
            body: JSON.stringify(...)
        });

On the server side, HttpServletRequest.getReader() returns a string <Unreadable> (this exact spelling, and without any other error or exception) instead of the JSON string sent by the client. Exactly the same happens for 'Content-Type': 'text/plain; charset=UTF-8'. The JSON string has about 150kB and at a glance, logged to the browser's console before being sent, looks like a single-line JSON string.

For tests, I sent the contents of a few-kB text field instead of the JSON string. This time, the server received an identical string without any problems. Only the JSON string could be sent for some reason. I tested if limiting its size helps. As it turned out, this worked:

body: JSON.stringify(...).substring(0,99999)

but this didn't:

body: JSON.stringify(...).substring(0,100000)

The following:

body: new Blob( [ contentsCallback().substring(0, N) ], { type: 'application/json' } )

works for N<100000. This:

const formData = new FormData();
formData.append('blob', new Blob( [ JSON.stringify(...).substring(0, N) ], { type: 'application/json' } ), 'test');
...
body: formData

works for N<99823.

It looks like there is a size limit somewhere. I do not know if it is on the client or the server side. This is a fragment of server's application.properties where I put various properties discussed here:

multipart.max-file-size=20MB
multipart.max-request-size=20MB
spring.servlet.multipart.max-file-size=20MB
spring.servlet.multipart.max-request-size=20MB
multipart.maxFileSize=20Mb
multipart.maxRequestSize=20Mb
server.tomcat.max-http-post-size=10000000
server.tomcat.max-http-post-size=100000000
server.tomcat.max-swallow-size=100000000

This is how the servlet is registered:

@Bean
public ServletRegistrationBean commandRegistration() {
    ServletRegistrationBean bean = new ServletRegistrationBean(
            new CommandServlet(), "/command");
    // final int SIZE_LIMIT = 20000000;
    // MultipartConfigElement mc = new MultipartConfigElement(TMP_FOLDER, 
    //         SIZE_LIMIT, SIZE_LIMIT * 2, SIZE_LIMIT / 2);
    // bean.setMultipartConfig(mc);
    return bean;
}

Uncommenting the commented part does not change the limit in question.

Here is Chrome data of fetch() which sends the full 150kB JSON (received as <Unreadable>):

Request URL: https://localhost:8443/?command=save
Request Method: POST
Status Code: 200 
Remote Address: [::1]:8443
Referrer Policy: strict-origin-when-cross-origin
Cache-Control: no-cache, no-store, max-age=0, must-revalidate
Connection: keep-alive
Content-Length: 0
Content-Type: text/html;charset=UTF-8
Date: Thu, 18 Mar 2021 09:32:00 GMT
Expires: 0
Keep-Alive: timeout=60
Pragma: no-cache
Vary: Origin
Vary: Access-Control-Request-Method
Vary: Access-Control-Request-Headers
X-Content-Type-Options: nosniff
X-Frame-Options: SAMEORIGIN
X-XSS-Protection: 1; mode=block
Accept: */*
Accept-Encoding: gzip, deflate, br
Accept-Language: en-US,en;q=0.9,pl;q=0.8,de;q=0.7,fr;q=0.6,ru;q=0.5,es;q=0.4,nl;q=0.3,cs;q=0.2,it;q=0.1,nb;q=0.1
Cache-Control: no-cache
Connection: keep-alive
Content-Length: 146111
Content-Type: application/json
Cookie: JSESSIONID=961B41156DCF6F7C57455AA871525198; JSESSIONID=72CCDD7A91C48660B9DD01041F4BEFBB
Host: localhost:8443
Origin: https://localhost:8443
Pragma: no-cache
Referer: https://localhost:8443/
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: same-origin
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4240.193 Safari/537.36
command: save

There is neither error nor warning in the console log. The limit is present also when Firefox is used or if I switch the HTTP server from Tomcat to Jetty.

Finally, I searched all sources and all jars (including the dependencies in the local Maven repo) for <Unreadable>. The string cannot be found there.

Can this limit be increased? How to send a large string to a SpringBoot application using the POST method? These answers suggest that practical limits for POST are much larger than 100kB.