Bonus Spell Slot don't add but removes

[Styrbjorn]

Favored Soul.
Whip with Bonus Spell Slot Favored Soul Level 3.
On equip, no extra spells level 3 added.
On unequip: removes one level 3 spell.

[DaloLorn]

You have to rest to get the extra slot... and yes, every time you unequip the item you lose a prepared spell.

It's a long-standing vanilla bug, nothing we can do about it on our own.

[Styrbjorn]

Ok, got it.