添加另一个答案，因为我遇到了同样的问题，但解决它的方式不同。
我也有一个请求日志过滤器，它 Package 了传入的请求，并以JonasPedersen在回答中描述的类似方式缓存了响应输入流。Sping Boot 从2.1.2.RELEASE更新到2.3.4.RELEASE
我将传入的请求 Package 在一个缓存请求 Package 器中，该 Package 器缓存输入流。对我来说，问题是由于某些原因（目前还不知道）request.getParameterValue（String key）方法返回null，即使 Package 的请求显然有一个非空的参数Map。简单地访问 Package 的请求参数Map就解决了我的问题……非常奇怪。
原始的 Package 器类，与Sping Boot 2.1.2.RELEASE一起使用：

public class CachingRequestWrapper extends HttpServletRequestWrapper {

    private final byte[] cachedBody;

    public CachingRequestWrapper(HttpServletRequest request) throws IOException {
        super(request);
        InputStream requestInputStream = request.getInputStream();
        this.cachedBody = StreamUtils.copyToByteArray(requestInputStream);
    }

    @Override
    public ServletInputStream getInputStream() throws IOException {
        return new CachedInputStream(this.cachedBody);
    }

    @Override
    public BufferedReader getReader() throws IOException {
        ByteArrayInputStream byteArrayInputStream =
                new ByteArrayInputStream(this.cachedBody);
        String encoding = StringUtils.isEmpty(this.getCharacterEncoding())
                ? StandardCharsets.UTF_8.name()
                : this.getCharacterEncoding();
        return new BufferedReader(new InputStreamReader(byteArrayInputStream, encoding));
    }
}

为简洁起见，省略了CachedInputStream类的实现。
简单地访问 Package 的请求Map似乎可以解决整个问题。对于Sping Boot 2.3.4.RELEASE，此版本有效（我删除了一些细节）：

public class CachingRequestWrapper extends HttpServletRequestWrapper {

    private final byte[] cachedBody;
    private final Map<String, String[]> parameterMap;

    public CachingRequestWrapper(HttpServletRequest request) throws IOException {
        super(request);
        parameterMap = request.getParameterMap(); // <-- This was the crucial part
        InputStream requestInputStream = request.getInputStream();
        this.cachedBody = StreamUtils.copyToByteArray(requestInputStream);
    }

    @Override
    public Map<String, String[]> getParameterMap() {
        return this.parameterMap; // this was added just to satisfy spotbugs
    }
}

我没有费心去深入研究这个问题，所以我不能说两个Sping Boot 版本之间发生了什么变化，或者在wrappers构造函数中访问参数Map解决了什么问题。

经过无数个小时的调试，原来是一个日志过滤器的问题。我是这样阅读请求的：

private static final class BufferedRequestWrapper extends HttpServletRequestWrapper {

    private final byte[] buffer;

    BufferedRequestWrapper(HttpServletRequest req) throws IOException {
        super(req);

        // Read InputStream and store its content in a buffer.
        InputStream is = req.getInputStream();
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        byte[] buf = new byte[1024];
        int read;
        while ((read = is.read(buf)) > 0) {
            baos.write(buf, 0, read);
        }
        this.buffer = baos.toByteArray();
    }

    @Override
    public ServletInputStream getInputStream() {
        return new BufferedServletInputStream(new ByteArrayInputStream(this.buffer));
    }

    String getRequestBody() throws IOException {
        return IOUtils.readLines(this.getInputStream(), StandardCharsets.UTF_8).stream()
                .map(String::trim)
                .collect(Collectors.joining());
    }
}

这消耗了来自内容类型application/x-www-form-urlencoded的查询参数。我的解决方案是不阅读application/x-www-form-urlencoded的输入流。

if (req.getContentType() == null || (req.getContentType() != null && !req.getContentType().startsWith("application/x-www-form-urlencoded"))) {
    // Read InputStream and store its content in a buffer.
    InputStream is = req.getInputStream();
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    byte[] buf = new byte[1024];
    int read;
    while ((read = is.read(buf)) > 0) {
        baos.write(buf, 0, read);
    }
    this.buffer = baos.toByteArray();
} else {
    buffer = new byte[0];
}

如果您也使用日志，则可能发生这种情况。参见https://github.com/zalando/logbook/issues/1077
显然，禁用表单日志记录可以修复它。

logbook:
  filter:
    form-request-mode: off