@-Max I think the user is reporting that it is possible to hatch a
lopunny egg by using soothe bell on Buneary egg.
Your post sounded as if the user was asking how to get a lopunny
from an egg.
@MeowMaid, are you sure it's not just cause the egg hatched after
you loaded the page? ^^ The icon doesn't change (or doesn't change
immediately - can't remember which) so you probably just evolved a
recently hatched buneary.
I just tried the bug with an unhatched Cottonee and a Sun Stone.
Indeed, the Sun Stone was was used up and I got the "Cottonee
evolved into Whimsicott" message. It has yet to hatch, but this is
definitely a bug.