The issue has been fixed for me now although I didn't get any form of response. Took approximately 48 hours after reporting on feedback assistant.

@DTS Engineer Hi Albert. I'm currently having the same issues. My build suddenly expired now I can't add a build to external testing and invitation mails don't get sent for internal testing for my 2 apps. Please I need help on this. Case ID: 103014992155 FB25400921

@DTS Engineer Please I'm currently having the same issues. My build suddenly expired now I can't add a build to external testing and invitation mails don't get sent for internal testing for my 2 apps. Please I need urgent help on this. Case ID: 103014992155