Ok, I gave him another chance to Kubuntu.
I can confirm that Oxygen-gtk theme is the responsible for this bug, if you run the program as this way:
$ OXYGEN_DISABLE_INNER_SHADOWS_HACK=1 ./theprogram (or ./lazarus)
the problem disappear, but there is another artifact, the scrollbars on designer are not painted, they are functional but there are only empty spaces where the scrollbar should appear. It must be another oxygen-gtk problem but I have not investigated too much about another command line workaround.
Instead, in revision 40585 and 40586 I modified the report designer to overcome this problem