Once you get into using table calcs, you need to do most filtering using table calcs as well.

In the attached I am using INDEX to display the last 18 values.

I displayed the value just for the sake of showing the value for demonstration purposes.  You don't actually need to display it, of course.

Just an explanation of what's going on here.

When you use a quick filter to filter your sheet, it literally discards rows from the underlying table.  If you have a table calc that needs all the rows to behave properly, you get garbage results when rows are deleted.

So I used a table calc as a filter. (So did Lukasz in the original solution.)  That leaves the underlying table intact, and that's why the displayed values of INDEX in the filtered sheet didn't start with 1.  All the prior months are still in the underlying table (so yes, it could result in carrying performance overhead on the sheet) but you need them for all the table calcs to work.