[Bug]: Mobs go to zero hearts but do not die

[Ellunex]

Modpack Version Used

1.5.4

What happened?

I'm not sure exactly when it started happening, but all of the sudden I became unable to kill any mobs. Hostile or otherwise. My attacks deplete their HP but no matter what they never die. The HUD shows them at 0 hearts but they never die. It might only apply to sources of damage originating from me, as I thought I saw other mobs finish them off no problem. However, the below information may indicate otherwise.

This is a strictly single player world on hard difficulty. I'm going to try to delete my player data in the save and see if that fixes it. Nevermind, after restarting I was able to kill mobs again. I could have sworn I tried that before though. However, the mobs that I already hit to 0 hearts that did not die continued to be unkillable, so I had to wait for them to despawn. The issue has not re-occurred since, but I suspect it may again eventually.

This might also be related. I once saw a mob, a Mimic, have 'NaN / 50' for its HP in the HUD. It killed me well before I did enough damage to it so I couldn't verify if it could be killed.

I didn't see anything unusual in the log during that time. I could include it if you think it may be important.

Screenshots (if applicable)

No response

Relevant latest.log

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Relevant crash report

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[kkylern]

I can confirm that this is a real, reproducible issue, but I don't know why it happens, I have read that MouseTweaks can be a problem, but disabling it is unfavorable. We installed NaNny mod on our server because we can't restart the server very often.