When using the Waxweaver and /killing a player, it will set AbsorptionAmount and Health to NaN, making the player unable to die or respawn fully and shaking the screen and making the player in a state of death and life. It's a little funny.
When punching other players it spreads, any damage will return them to that state of undeath, and if only the Health or AbsorptionAmount are fixed and not both of them, the bug will not fix.
If punching another player in this state, that player will also be trapped into that state until the player data is fixed manually.
Version is 1.20.1, modded fabric.

I am unable to reproduce this issue.
Can you please perform isolation testing with only Simply Swords and its required dependencies?
If the issue does not occur in isolation, it's likely to be a bad interaction between mods, and you would need to identify the conflicting mod.

If i had to wager a guess, it may be an issue with bad interaction between mods if this is not a replicable issue, I know the modpack I use is pretty big. In that case, I'm not sure which mod it'd be? I'd need to sift through all 50~ mods, if that's something you wanted me to do to identify the issue.
Thanks for letting me know it's not baked into the mod itself, though! That's where my primary worry was.