If the order of the positions in the table changes with each price entry, you have probably activated a sorting e.g. on the unit price. You can recognize this in older versions by a triangle in the column, here in the picture e.g. at EP.
In version "GAEB-Online 2021" this is marked with a special symbol.
To solve the problem, right-click on the column header and then select Delete Sort.
See also the chapter Sort by price.