Thanks for reaching out and sorry to hear that you're having trouble in using alt key to change a part of the shape in XD. we already have a bug logged for the issue, the team is working on it and a fix is underway. You should be able to see this issue resolved in the upcoming releases. please download the latest version of XD and enjoy the new features.
Please feel free to update this discussion in case you need further assistance and we will be happy to help.