I encountered this bug long time ago, but this happened when I had custom shivering isles races mod in 2016. For today, I installed only UOP with DLCs support and Character Overhaul 2.03 with custom UOP-compatibility patch for it.
But I decided to do some google research, and found out, that this issue was even existing on vanilla PS3.
Here's examples of other users encountering this bug:
https://forums.uesp.net/viewtopic.php?f=6&t=19022
https://help.bethesda.net/#en/answer/17813
This makes me think, that this bug was not noticed by UOP developers, or it is might be not so easy to be fixed.
I use steam version with all DLCs enabled.
Here installations I use (I didn't forget to sort stuff by LOOT):
OBSE -
https://www.nexusmods.com/oblivion/mods/37952
UOP -
https://www.nexusmods.com/oblivion/mods/5296
UOP Shivering Isles -
https://www.nexusmods.com/oblivion/mods/10739
UOP DLC -
https://www.nexusmods.com/oblivion/mods/9969
Blockhead -
https://www.nexusmods.com/oblivion/mods/43752
Oblivion Character Overhaul -
https://www.nexusmods.com/oblivion/mods/44676
OCO UOP-compatibility Patch -
https://www.nexusmods.com/oblivion/mods/48136
I might can be wrong if this bug still was not noticed, then I'll apologize for that. But if it was not noticed for real, this bug is quite an annoying issue, that appears randomly in intense combat on high difficulty, where you need to move quickly much, and it ruins your melee combat, so you can be easily eaten by rats on tutorial level.
I'm not lucky now to reproduce this bug, tried to equip weapon going down on terrain, or reloading saves on attacks, still not lucky to encounter it, but believe me, this bug somehow still happens somewhen.