Karthus "E-Defile" won't charge Tear of the Goddess!

Have encountered this bug in multiple games on patches 6.17 and 6.18: Activating the E-spell ("Defile") on Karthus and deactivating as soon as possible again won't charge Tear of the Goddess and won't add maximum mana despite costing mana upon toggling E. The description for the Tear says it adds 4 maximum mana on spell cast/MANA EXPENDITURE and does not do so with Karthus' E. This continued to happen, meaning I couldnt charge the tear despite toggling E on and off multiple times without casting other spells for 10s or more.