Hi!

I am no dev, just putting my thoughts out here.
Only
this one specific cat disappears like this? Or other, different animals, too?
If only this cat, maybe let it be? It is only part of the Nehrim Expanded mod, not the base Nehrim game, so the Nehrim original files will not help you, I think. Unless you reinstall the whole game from the beginning.
If all cats are bugged, then you have a bigger problem.
In either case. You should download TES4Edit (
https://www.nexusmods.com/oblivion/mods/11536/). When you load all your mods in the programme, you will see the conflicts between all the mods and Nehrim expanded (usually shown red). Maybe you can find what causes the problem.
Or you can try disabling all your mods, aside from Nehrim Expanded, to see which one causes the problem with the cat or cats. If you find the mod responsible, you can remove it, to fix the problem?
However. Even original Oblivion had some weird issues with dead animals, dead body collision, etc. It might not be Nehrim itself.