Small objects don’t have corners widgets displayed.

Take a look at the test file with small 3 rectangles.

The largest one does have its widgets, but two smaller ones don’t.
Once you scale them up to 110%, the middle one now has them.
Another scaling up — all three now have them.

Yes, these are very small, hardly larger than 1 pixel! But what a difference it makes for a user? None, really. Ai is capable to display all of them and allow tiny transformation, like this scaling up. Why does rounding some corners should be anything different?