Since this error has only occurred for a few days and I have never encountered it before, I assume that it is related to the last AI2 update. @ewpatton
I went through all the versions of App Inventor from nb181 (when multiselect was released) through nb183b (released yesterday). I was able to reproduce this issue in nb183, but not nb183a. I did quite a bit of work between nb183 and nb183a on the multiselect functionality based on reports from @SteveJG and @ABG. The bug was that having the screen selected along with dragging a component into a TableArrangement would result in the Screen inheriting the column and row properties erroneously. I’ve submitted an additional patch that will remove these properties when the project loads (pending review).