My experiments were also done with text flow, but I seem to have focused on a different aspect. I did see problems with text flow when two text boxes touched the same margin, but when none of the text boxes touched any margin everything seemed to be fine. I didn't move boxes around much, though.
Today I tried to reproduce your findings, but strangely I had no problems moving boxes around. I suspect the problems you saw may be caused by KWord not handling cursive fonts correctly (I used an upright font today). It is also possible that the differences we experience are due to the fact that we use slightly different versions of KWord, but then it is odd that you should see problems that I don't, since your version is the most recent.