Are you sending those tests emails from a production instance? Did you use template emails created by Deliverability for branding? (sender and reply-to correctly set). If not sending from a production instance, you are maybe sending those tests with default sender domaine (adobe.com) and it can be an issue for some ISPs.
Is there any mid-sourcing in this setup? You may look on mid-sourcing instance where stands your tests. Maybe an issue arised and those tests can't be sent out of the platform, and feedback will not be reverted to marketing instance immediately, but almost an hour after.