Noel thanks for reply. I think i understand what you are saying but 2019 seems different from previous versions. I don't think I am rendering song.
i open new song from desktop called 'untitledsong.sgu'
I record vocal and everything plays back okay. there are 2 new files called 'tempmain.wav' and 'tempmain.bt1'.
I do not 'save'
i then record audio again and when I play the song back it plays the first recording. There are now 2 wave files called tempmain.wav and temp-rec.wav. If i play the wav files only I get the first and second audio recordings.
If I 'save' (I don't think this is render) the song then I get another file called 'untiitledsong.wav' If i play this as wave file it plays only the vocal and no backing track.
After clicking 'ok keep take' I am also occasionally getting 'error file not found' and one or more new files are made which is full of symbols and unusual characters.

I have windows 10 pro with bitlocker. When I see the characters and symbols I am wondering if there is an issue with bitlocker
Thanks