If a cell spans the whole table and its height is bigger than the sum of the heights of all the other cells, then the computation of Knuth elements by the merging algorithm is wrong. At the end of the sequence there will be a box with negative width. That's because ActiveCell.getRemainingHeight returns 0 if the active row is not the last row spanned by the cell.
Created attachment 21023 [details] FO file showing the problem
Fixed in rev. 619854
batch transition pre-FOP1.0 resolved+fixed bugs to closed+fixed