Why Don't Cells Retain Their (currency) Formatting?

I'm building spreadsheets in a foreign currency (Brazilian Real -- R$) and in the same manner that I've formatted cells for years, knowing that I'll want the entire column to reflect a given text or number format, I click on the column's designated alphabetical letter at the top of the column, which highlights the entire column, click the cells inspector button (in the Inspector), set the Cell Format pull-down menu to Currency, and set the Symbol pull-down menu for Brazilian Real (R$). Any numbers in the column are immediately formatted for the Brazilian currency, including, I assume the numerous blank cells below where I'm actually working in the spreadsheet. I always have an additional 50 blank rows, pre-formatted, so as I add data to the rows, all the cells format accordingly for both text and numbers.

This apparently appears to be working because in general, as I add data to the spreadsheet, all the cells format according to how I've formatted their columns. After formatting a column, I always save the document. One would think that would lock in any given column's formatting from that point forward.

Every once in awhile, however, for reasons unknown to me, when I enter a numerical value into the currency column, it does not format as Brazlian Real (R$) and I have to reformat the cell again. Usually I just click the entire column (again) and reformat the entire column. This happens often enough to interrupt workflow and be an annoyance.

What am I doing wrong? Why aren't these cells retaining their Currency formatting?

Thanks for any and all solutions!