When attacking a large group of guardians with a sword, it seems like the sweeping edge of the sword also results in thorns damage being applied, unlike in vanilla. This means that if you hit enough guardians you instantly die, but it gives no death message in the chat, and you just teleport to your spawn point with all of your items but no XP (keep inventory is not enabled).
Can Reproduce in version 4065 but no longer in version 4066 or 1.20.6.
This was caused by the commit 06d915943c9, which got reverted a version later in 45d6a609f96.