Seems like sometimes (not always) and usually underground (like where we find Withers or owlbear), when we throw spears or handaxes etc, it seems to be doing "crushing damage" on the user before it is throw and then it does the normal damage + crushing damage to the target as well. This damage is significant and can kill the player characters immediately.
Tried throwing with companions, they don't have this issue, only PCs, also if an item is thrown own and it has inflicted damage, if picked up and throw again it does not self-inflict in the future.
I am not sure how this is coded in but it is possible that when we load into the game the items are "loaded into on demand" and maybe the Z-axis is not correct and the items are literally falling into our hands from a distance and causing damage.
Never had this issue in patch 8, maybe this is something introduced in patch 9.