Something smells weird with ODB++ export or import. I'm not sure which.
Here are my steps:

Create a board that looks like Pacman, i.e. rounded and concave, using a couple of arcs.
Create a copper pour using "by border", with 1mm clearance.
Export as ODB++ using the default settings.
New file / clear the workspace.
File / Import / ODB++
Select the file you just created.
Observe the weirdness in the copper pour.

I tried playing with the concave/convex slider and can't find a way to make it do what you'd expect it to. I don't have any other software that would help me determine whether it's the export or import that's weird. I'm using the latest version of DipTrace, 5.2.0.1

Thank you for your bug report. The issue can be reproduced following the steps you described. I’ve added it to our internal bug tracker.