The mobs I spawn can have a randomized healthpool, ranging between 50 and 75 ish from what I have noticed.
With the healthmultiply set to 2 I would expect it to multiply the health of every unique mob's base health.
However, I have found that it sets every mob's health pool to a specific number, in this case 60.
So if a mob spawns with HP greater than 60, it causes a weird glitch where the active health is greater than the health limit.

I am also not sure as to where it got the number 60 from, as either 50 or 75 * 2 is never 60.
Maybe it took the baby's health.

It can't be the babies' health, since mobs with much larger health suffer the same fate.

Entire spawn.json

[
    {
        "dimension": 0,
        "mod": "rebornmod",
        "mincount": {
            "amount": 25,
            "mod": "rebornmod",
            "perchunk": true
        },
        "result": "deny"
    },
    {
        "mod": "rebornmod",
        "healthmultiply": 2
    }
]