Live-build (buster) builds stretch No buster.

[charlie101 <charlie101@journeysend.co.uk>]

I found this  question has already been asked here: 
https://lists.debian.org/debian-live/2019/10/msg00002.html.

Unfortunately there was no accompanying answer.

Can anyone tell me what the deal is with live-build building 'stretch' 
images instead of buster, and how one might fix this, assuming there is 
an easy fix.  Or should I report it as a bug?

Thanks in advance

charlie