Bug? AsyncStreams buffer length not accurate

hi, im doing serial com using bluetooth and use AsyncStreams to read and write data. the problem is, the number of buffer length is not accurate. for example, when i sent string data "123456", should be the buffer.length = 6. but when i print the string and buffer.length it shows string = 6, and buffer length = 1.
i can see before it display string = 6, it changed from 1 to 6 in a very fast time. does the problem cause by the phone or by the IDE bug?

This is not a bug or a problem in your phone.

How are you sending the string?
When you send a message it might be split because of the Bluetooth buffer.
See AsyncStreams tutorial.