This information applies to version(s): 6, 7
For DxO PhotoLab 6.2, DxO PureRAW 2.5, and all previous versions, when using the distortion correction, the corners of the resulting images were slightly cropped, resulting in the maximum rectangular image area.
Beginning with DxO PhotoLab 6.3 and DxO PureRAW 3 it is possible to gather all of the images projected by your lens. This results in black areas, which are not part of the image and could at first sight be misunderstood as distortion. However, this is just the outer shape of the image, whereas lines as part of your scene will be perfectly straight. The crop at distortion can be changed for each image/batch of images. In the palette, Distortion > more option you might want to select the option "constrain to image" which is what you have been used to. Then add this setting to your preferred custom preset or create a new one.
The presets “DxO Styles - Natural” (version 7 only) and “DxO Standard” have the crop option “constrain to image” activated, while the preset “optical corrections only” has the crop option “constrain to image” deactivated and thus shows the effect described above.
BTW: black areas are not an active part of the image, ReTouch won't copy to those areas.