Thanks for finding the time to report this problem. This is a known issue and we are working on solving it! Please stay tuned for the updates.
In the meantime, we can suggest you a workaround, please enable the Node tool when you want to apply Corner radius to the object created using the Pen Tool. In this case, this property shouldn’t disappear. I also attached a short screen recording for you.
Hope it works for you.
Happy to announce that we recently released an update 4.5.0 that fixes the issue you reported. May I ask to update the app and check it for us? Hope this problem will no longer bother you
Please let us know if you still experience any issues.