Adding a row to a table seems to break it

[msdobrescu]

Having a document with an "irregular" table with regard to the cells sizes per row, leads me to the following issue:

+--------+------+----------------+--------+
|        |      |                |        |
+---+----+---+--+--------+-------+--------+
|   |        |           |                |
+---+--------+-----------+----------------+

Adding a row using add_row function creates approximately this:

+--------+------+----------------+--------+
|        |      |                |        |
+---+----+------+--+-----------+-+--------+
|   |              |           |          |
+---+----+------+--+-----------+-+--------+
|   |    |      |  |           | |        |
+---+----+------+--+-----------+-+--------+

The rows report 8 cell elements of type _Cell, but according to the references, there are 4 objects twice.
The Borders are missing with the exception of the last one at the right.

What is the correct way to add a row and get either a row like the previous one, at least having 4 cells and the right borders?
There must be a procedure for this!