As above, when on a single player world these attributes apply multiple times (at least twice) giving mobs ridiculous attributes.

However, when using same configs with a separate server, the attributes appear to apply correctly.

Seems to apply to both the add and multiply variants. Tested with MC Eternal mod pack on both mod versions.

Had a chance to spend a bit more time testing things for this. Turns out it's an optifine thing. Without optifine, both mod versions work as expected.