According to Serebii:
This special Poké Ball is used for capturing Ultra Beasts. When capturing Ultra Beasts, the Capture Rate bonus is a massive 5. However, when used on a non-Ultra Beast, the Capture Rate bonus is 0.1, making it even harder to capture standard Pokémon.
CatchValue = (((3 MaxHP - 2 HP) (Catch Rate 0.1) / (3 Max HP)) Status Modifier
The Beast Ball's catch rate for regular Pokemon is literally 0.1; that is, a tenth of the chance of catching a Pokemon with a regular PokeBall. That's unfortunate.
Hope I helped. :)
Source: Link above