BASE64DecoderStream fails with InputStream.readAllBytes

I have a small snippet which reads the attachments from an IMAP inbox:

BodyPart bodyPart = content.getBodyPart(i);
        
if(bodyPart.getFileName() != null){
    logger.info("Retrieving file {} with size: {} MB", bodyPart.getFileName(), bodyPart.getSize()/1024D/1024);
    Object innerContent = bodyPart.getContent();
                    
    switch(innerContent.getClass().getCanonicalName()){
        case "com.sun.mail.util.BASE64DecoderStream":
            BASE64DecoderStream innerContentStream = (BASE64DecoderStream) innerContent;
                            
            final byte[] actualContent = innerContentStream.readAllBytes();
            //final byte[] actualContent = IOUtils.toByteArray(innerContentStream);
                            
            logger.info("Downloaded {} bytes", actualContent.length);
            files.put(bodyPart.getFileName(), actualContent);
            break;

        default: 
            logger.error("Unimplemented content type: {}",innerContent.getClass().getCanonicalName());
                            
    }

With InputStream.readAllBytes() the filesize is always 8192 bytes (one buffer), whereas Apache's IOUtils.toByteArray(InputStream) reads the files correctly.

I actually wonder why this is the case. Did anyone run into this issue as well?

Java versions:

openjdk version "11.0.5" 2019-10-15
OpenJDK Runtime Environment AdoptOpenJDK (build 11.0.5+10)
OpenJDK 64-Bit Server VM AdoptOpenJDK (build 11.0.5+10, mixed mode)