Aye, I tend to agree that Bill does seem to be the most likely suspect.
Perhaps a bit odd given that he only just got the twilight sword unless it it is what defeats the God Of Hammers.
I also wonder if that swords power could repair stormbreaker?
As to what is wrong with asgard's magic
1. Ancient Gods have tampered with it
2. the g.o.h. has tampered with it
3. Those Who Sit in Shadow have tampered with it.
4. Odin is way past due for an Odin sleep.
5. The Odin power does not like being split/shared between Odin and THOR.
When THOR inherited the power, ODIN was dead. Now THOR sits on the Throne, presumably with the power but Odin lives.
6. ODIN's own issues have weakened his power and thus the enchantments on the hammer.
7. Dr Strange repairing the hammer when it was damaged may factor into this also
8. THOR's ending of the Ragnarok cycle when he previously was KING. This has caused some type of mystic disruption and the only way to restore things is yet another Ragnarok
It occurs to me that the sword Bill has would certainly be capable of a scorched earth effect.