+FIDX not working consistently - BUG?

[LucyI]

I have this as part of my FileDir inner template:

class="th fix-height [[+fidx:is=`1`:then=`clearing-feature`:else``]]"

Sometimes it applies the class clearing-feature to the first item, but sometimes it applies it to the 4th or the 5th. Can't seem to find any reason. Have tried clearing the cache, and deleting and uploading new files. No luck.

[LucyI]

Have played around with this some more, and every time I change the group of files in the directory, the class is applied to a different number item in the group. It always picks just one item, but it seems to be randomly picking one out (sometimes the first item, but also sometimes the 3rd, 4th, 5th, etc.)

[LucyI]

Just to reiterate: If I turn the sort off then +fidx counts as you would expect, with number 1 being the first item. If I turn sorting on, then +fidx assigns number 1 to any item in the array, not necessarily the first item, I assume whichever item was number 1 before the sort happened...?