Hi Mateusz ,
I'm really sorry that you are running an issue. I was able to reproduce the issue and logged a bug internally on your behalf.
For now you can try one of the workaround for your use case:
Switch to Code view and change the rx and ry values = 0.5 instead of 1 for oval.
<svg data-gwd-shape="oval" preserveAspectRatio="none" viewBox="0 0 1 1" class="gwd-svg-9ch6 gwd-gen-15w4gwdanimation">
<ellipse cx="50%" cy="50%" rx="0.5" ry="0.5"></ellipse>
and Switch back to design view.
Workaround 2 :
Create a Div instead of an oval and in properties panel, edit the top left border radius with keep identical to make it a circle.
I hope this helps!
(Google Web Designer Team)