Hello
I can reproduce this also on win 7-64 in QCAD 3, agreed the 'padding' as you say is absolutely perfect and this does result in a tightly cropped fit when exporting to a bitmap image, maybe a 'padding' border size option could be added to the export dialog here!
For now a workaround you could use is set out below, it`s not perfect but it may help you out for now
1.Set up new layer 'Marker' with color set to something you can see well e.g. Yellow.

- bitmap export - workaround 1.png (32.46 KiB) Viewed 887 times
2. Working on this new layer 'Marker' draw small circles where you need the border 'padding' to be.

- bitmap export - workaround 2.png (23.24 KiB) Viewed 887 times
3. Finally select these added circles and in the property editor change the radii to ' 0 '.

- bitmap export - workaround 3.png (18.33 KiB) Viewed 887 times
4. Export your file to bitmap and you should get a better result

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- bitmap export - workaround 4.png (15.66 KiB) Viewed 887 times
Hope this comes in useful for now.