I was killing wither skeletons with Smite V and Strength II diamond sword. I saw that I had 13 kills. I killed a wither skeleton and checked my stats: still 13. Then I killed another: now 15.
To reproduce:
Spawn in a bunch of wither skeletons.
Get a Smite V diamond sword (netherite probably works, too)
Apply Strength II
Kill the wither skeletons and count how many you have killed
Check your stats.
Repeat 4 and 5 until you notice a discrepancy.
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