Navbar Bugada só em tela pequna

você pode alterar para

.justify-content-start
.justify-content-end
.justify-content-center
.justify-content-between
.justify-content-around
.justify-content-sm-start
.justify-content-sm-end
.justify-content-sm-center
.justify-content-sm-between
.justify-content-sm-around
.justify-content-md-start
.justify-content-md-end
.justify-content-md-center
.justify-content-md-between
.justify-content-md-around
.justify-content-lg-start
.justify-content-lg-end
.justify-content-lg-center
.justify-content-lg-between
.justify-content-lg-around
.justify-content-xl-start
.justify-content-xl-end
.justify-content-xl-center
.justify-content-xl-between
.justify-content-xl-around

Para ver qual se encaixa mais. Não sou bom de design então não vou palpitar qual fica mais legal rsrsrs

Para saber mais:

https://getbootstrap.com/docs/4.0/components/navbar/

https://getbootstrap.com/docs/4.0/utilities/flex/#justify-content

Espero ter ajudado!!!