The changelog of version 1.12-3.9.17 says that this is fixed, but it seems to still occur when the "onjoin" condition is used with conditions that multiply or add health/speed/damage.

This was most easy to replicate by closing and reloading a singleplayer world with the relevant entities nearby.

This is an example of my script that resulted in this issue:

[
    {
        "mob": "minecraft:wither_skeleton",
        "onjoin": true,
        "dimension": -1,
        "healthmultiply": 2.0,
        "damagemultiply": 1.5,
        "helditem": "embers:sword_lead",
        "result": "default"
    }
]

To expand on this, the "helditem" rule doesn't seem to work without using "onjoin," which is why I am using it at all in this example.

Rules like "armorhelmet," "armorboots," etc seem to work fine without the need for "onjoin."