Zend certified PHP/Magento developer

Why does chart formula change when using dynamic formula in Name Manager?

When I am trying to create a dynamic chart, I use name manager to set the axis. The formulas in name manager will either be an offset function or an index function, an example would be:
=DATA!$B$14:INDEX(DATA!$B$14:$B$1000,DATA!$C$11) this formula gives me the data i need when i change the dates and length of the dataframe. The problem is, once in a while, when i refresh the workbook, I notice that the chart doesnt update. When i go into the chart data and see the formula, i see that the name manager formula is replaced by a weird formula along the lines of ={r_2}. It is always some variation of this, like sometimes it’ll be 1, or 3 or 8 instead of 2 at the end.

My fix so far has been to just delete that formula and replace it again with the name manager formula. But why does this happen? How can I prevent it from happening?