edgin has a lesser singer shaft (spear) that is supposed to grant 1 bonus L1 bard spell and 1 bonus L2 bard spell when it is wielded. (amongst other things)

according to a bard he temporarily sold it to, it does not work this way but instead takes away spell slots when held. i tried it on a module and it worked correctly.

i'd like to check this with the dms in game if possible.

If there's any bug here it's a Bioware issue. It may also be that unequipping the item removes spell slots.

For bards, equipping weapons that add Charisma, enough to grant extra spell slots, will always remvoe one prepared spell of the respective levels when unequipped, means when you equip and de-equip them often enough you get some spells to 0 uses without even casting a single time. Must be it is the same for bonus slots, which is the problem why I generally hate such boni on items I cannot keep permanently equipped (like weapons and armor since I might not want to look so battle-ready all the time). It's a common Bioware bug methinks.