Hi,
I just tested this again to be sure, but everything works correctly here, and with your settings the child rotates around its own Y axis as it should. Could you replicate this in a barebone project, upload it somewhere, and send me a download link via Personal Message (by clicking on the balloon icon under my avatar)? That way I can see what's happening.
P.S. also, be sure you're using the latest version of DOTween Pro.