indexing problems with arabic

[nutztier]

I am currently correcting an arabic text. While building an index I encountered some problems which I would like to understand better before I start all over again.
By accident I added an index-mark for the letter combinations tha-mim and ra-ha and had OpenOffice mark these combinations even if they are not complete words. So far my stupidity.
But now I found that OpenOffice scrambled the writing direction of these letter combinations from right-to-left (tha-mim, ra-ha) to left-to-right (mim-tha, ha-ra) midword! It did the same thing with some other words (akkem instead of mekka) but not for all?
Can anybody give me a hint why this is and what made the difference?
I tried different fonts and file-formats and none of them worked.
I put some screenshots on my website to illustrate:
http://www.bjoernbentlage.de/pictures/index.html
thanks[/img]