If an Arrow of Harming's base arrow damage is enough to kill while you're holding a Totem of Undying, the Harming damage is dealt after the totem activates. You can even bypass the totem entirely with only a Harming II Arrow.

Steps to Reproduce:

• Run "/kill" to ensure you have full health and no player-assigned attributes.
• Set the "naturalRegeneration" gamerule to "false" to avoid yourself from regenerating.

• /gamerule naturalRegeneration false

• Obtain a totem of undying, a bow, and an arrow of harming with instant damage 2.
• Hold the totem of undying in your offhand and the bow in your main hand.
• Switch into survival mode and teleport yourself 23 blocks upward so that you have half a heart of health remaining.

• /tp @s ~ ~23 ~

• Look upwards and shoot yourself using the bow and harming arrow.
• Take note as to whether or not harming arrows can bypass the totem of undying.