This issue has now been fixed! Thank you for your report!
There was indeed a bug in Composer that caused border radiuses and widths not to show up correctly on the canvas (unless you were defining each of them individually). Luckily, this problem had no effect when running your app. It just affected how the app was previewing while editing. There is no actions you need to take except to reload your browser window (or if necessary, clear the browser cache).
It’s specifically for the Appgyver Beta program, so I’m unsure if it’s an open-door policy there – but you would have a fast-track to reporting new issues and the like on Slack. I don’t think it’s my place to post the link, but if you want in on the beta, perhaps, @Harri_Sarsa could invite you.
You were right, there was another bug related to absolutely positioned components! This issue should now also be fixed! Ensure that you reload your browser, and let me know, if you still encounter this or similar issues!