OP is going to have to use console commands if he wants to solve or diagnose this further. All the usual scenarios have been mentioned - mods, unique bug to OP, some base game dmg source, enemy spawn, etc.

1. enable console

2. OP says he can see the boars explode so he knows the exact spot where one died - goto that spot and stand there

3. build small 1x1 or 2x2 square cage

4. use console command to spawn boar 1 (or anything that takes dmg, a nek, whatever

5. jump out of the spot and now sit there and wait 1-2 full game days/nights watching what happens

if the new spawned creature explodes and dies, something is either wrong with that spot - some dmg source you can't see or OP has some hilarious yet sad bug unique to his world / savegame

if the new spawned creature doesn't take any dmg, then I have no other ideas since OP has said tried moving to new spot and says has same problem. that point, either live with unique bug or give up on Valheim. Not being mean but after exploring all diagnosis options, that's what it comes down to.