@jdebouil We have completed analysis and concluded It is not an Aspose.Words bug.
inSimple.zip (13.9 KB)
After re-save we will see two rows in the LibreOffice app.

Libreoffice can not process correctly cases when cells have both vertical and horizontal merge. Libreoffice application converts horizontal merged cells into the one combined cell.
At the same time when a souce DOC document has cells with vertical and horizontal merges then it looks like the Libreoffice skips vertical merge.
However it is possible to update AW model and obtain expected output for the problematic document:
Document doc = new Document(@"in.odt");
Table tbl = (Table)doc.GetChild(NodeType.Table, 0, true);
foreach(Row row in tbl.Rows)
{
double cellWidth = 0;
for (int i = row.Cells.Count - 1; i >= 0; --i)
{
CellFormat cellFormat = row.Cells[i].CellFormat;
if (cellFormat.VerticalMerge == CellMerge.None)
continue;
switch (cellFormat.HorizontalMerge)
{
case CellMerge.None:
cellWidth = 0;
break;
case CellMerge.First:
cellFormat.HorizontalMerge = CellMerge.None;
cellWidth += cellFormat.PreferredWidth.Value;
cellFormat.PreferredWidth = PreferredWidth.FromPoints(cellWidth);
break;
case CellMerge.Previous:
cellWidth += cellFormat.PreferredWidth.Value;
row.Cells[i].Remove();
break;
}
}
}
doc.Save(@"aw_out.doc");
The code above replaces merges with one the combined cell.