Steps to Reproduce:

1. /give @s command_block
2. /gamerule commandblockoutput false(to prevent the message from flooding, unnecessary)
3. Place a command block, input"/summon elder_guardian_ghost", and put a button on it
4. Place another command block, input"/tp @e[type=elder_guardian_ghost] ~ ~1 ~", set the block type "Repeat",  put a lever on it, and turn the lever on.
5. Press the button to summon.
6. Take a wooden sword out, and aim at the shadow (the living entity is invisible) on the command block. [Phenomenon 1]
7. Summon another one. Throw splash potion of healing a few times. Repeat step 6. [Phenomenon 2]

Phenomenon 1: Four strikes can kill it, which is work as intended because wooden sword cause 5 health at a time, and elder guardian ghost is 20 in health.

Phenomenon 2: It needs more than four strikes, which is a bug, meaning that elder guardian ghost's health can be more than 20.

Unfortunately, I don't know how to prove the max health is 3.4×10^38 without addon.

I guess the bug originates in the negligence of the upper limit of health in elder guardian ghost, because the number 3.4×10^38 is the upper limit of Single number.