I have read check documentation: https://checkstyle.sourceforge.io/checks/javadoc/summaryjavadoc.html
I have downloaded the latest checkstyle from: https://checkstyle.org/cmdline.html#Download_and_Run
I have executed the cli and showed it below, as cli describes the problem better than 1,000 words

From: https://google.github.io/styleguide/javaguide.html#s7.2-summary-fragment

the fragment is capitalized and punctuated as if it were a complete sentence.

Code:

class Test {
  /**
   * adds an element to the list.
   *
   * @param element the element to add
   */
  public void add(String element) {}
}

Cli:

$ java -jar checkstyle-10.26.1-all.jar -c google_checks.xml Test.java 
Starting audit...
Audit done.

Formatter:

$ java -jar google-java-format-1.28.0-all-deps.jar Test.java > FormattedCode.java
$ diff -Naru Test.java FormattedCode.java
None

The Javadoc starts with “adds an element …” (lowercase a) and does not trigger any violation.
According to the style guide, this should be reported as a violation: the first word must start with a capital letter.
This is a false negative in SummaryJavadoc.

We tried configuring:

<module name="SummaryJavadoc">
  <property name="forbiddenSummaryFragments"
           value="^@return the *|^This method returns |^A [{]@code [a-zA-Z0-9]+[}]( is a ) |^\s*[^A-Z]"/>
</module>

Result:

Starting audit...
[WARN] Test.java:2: Forbidden summary fragment. [SummaryJavadoc]
Audit done.

Perhaps |^\s*[^A-Z] could be added to the google_checks.xml so that lowercase beginnings of Javadoc summaries are flagged automatically.