The bug

Dispensing an item with special behavior (e.g. shears, flint and steel) only sometimes plays the sound of using that item in your hand. In addition, the "dispenser success" click is sometimes (but not always) played on top of the item usage sound.

Only item usage sound played

• Firework rocket (plays both bow firing and firework launching sound)
• Arrow, snowball or potion (plays bow firing)
• Fire charge

Both item and dispenser clicking sound played

• Water, lava or fish bucket
• Shearing sheep
• Dispensing armor onto armor stand

Item usage sound not played (only plays clicking)

• Dispensing an empty bucket to collect water or lava
• Flint and steel
• Dispensing armor onto players (see MC-94060)