Its observed that with early access nightly builds of jdk-18-ea9 the conformance tests validating the charAt method of StringBuffer/StringBuilder are failing for the case -
Throws:
IndexOutOfBoundsException - if index is negative or greater than or equal to length().

Please find the attached standalone code snippet to reproduce the issue.

This looks a regression caused by the fix - https://bugs.openjdk.java.net/browse/JDK-8270160