There is a rounded corner sizing item in the Style tab when you select an object, but it does not actually change the rounded corner size unless you are using the rectangle, polygon, or star tool.
If this is a specification, it would be helpful if you could tell us the new way to do this, as it is at least a different behavior from previous versions.
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.