for (var k = 0; k < textframe.tables[j].rows.length; k++)
thank you for your help.
Check autoGrow property for cells of this table (should be = false)
If not insert a line:
textframes[j].cells.everyItem().autoGrow = false;
before those one with height modification.
Thank you Jarek!
My solution actually worked, it wasn't that clear when i did the test.
But thank you for your advice. It will make my script more accurate