Hi Benjamin thanks for posting. It sounds like your shapes and lines have a default style set. Setting a default style for an object causes any new shapes added to the document to take on the same style. For more information about this I recommend checking out the Shape Styling FAQs section of our Modify Format and Style shapes article. 

To change the roundedness of your shapes select the shape click on the Shape Options icon in your toolbar and toggle the roundedness as needed. You can then right-click on the shape you just changed and select 'Set Default Style' as shown in the GIF below. 

Hope this helps! Feel free to let me know if you have any questions.