In my testing, I have only used custom armor with armor values exceeding Netherite, Netherite, and Diamond armor.

For consistency purposes, I spawned a zombie in hard mode.

When being hit by an entity, the initial hit in these armors will always be less than the following hits damage on the player.

This screenshot shows a player at full health being damaged by a zombie while wearing full Netherite armor. It reflects that the player initially took ~.64 of their health as damage. The following hits took exactly 1.0 of the players health.

This screenshot reflects the same disparity but with full Diamond armor. Here the initial hit took ~.67 of the player's health, while the following hits took 1.0.

This is likely a small part of a very large issue when it comes to damage calculations being performed in Minecraft. It appears that after the initial hit, the game no longer takes into account the player's armor/armor toughness and resorts to a default damage value. I will make a separate bug report for this, as this drastically impacts gameplay and prevents armor with values above Netherite armor from functioning properly in addition to the lack of armor toughness being applicable to custom armor.

PLEASE NOTE: Due to changes to the EntityHurt script event, the attached pack only works in 1.19.70+