Dropdown range changes when re-opening file

[Joanne80]

I have a list of expenses on a certain project. I allocate each line to a certain type of expense by using a dropdown list.
When re-opening the file, the first few lines were good:
dropdownOK.jpg

But after a few lines, the dropdown range changed, although it keeps the selection I made in the previous version:
dropdown_wrong.jpg

I have seen that the data validation list changed to =$A$2:$A$17 instead of my separate list =Dropdown!$A$2:$A$17 but cannot understand why excel has made this twist.
Each month I update this list, which means adding lines to the list, both inserting them at the bottom, as in between other lines. I can't see any pattern, because it kept the range of the first 6 lines, when the previous version already had 50 lines or so. It does not only happen to the inserted lines, for example.

I can rectify by re-entering all data, but want to understand what is happening, to be able to avoid this in the future.

[Fotis1991]

As i understand your list is in another sheet-tab. Isn't it?

Then use a Name for your list.

Also pls do.t upload pictures. Upload a sample workbook.