Playing on iPhone (I haven't tested this on PC), when you have floating mana from interdimensional web watch which can only be used to cast spells in exile and floating mana from normal sources and you cast a spell from exile, the game will automatically use the mana from normal sources instead of giving you the option to spend the mana from interdimensional web watch. I made sure to have autotap disabled and even tried full control. This is a significant issue for the deck I play and am hoping this can be resolved before the standard qualifier coming up.

(edit - I tested this on PC and it works fine on PC so this appears to be a bug playing on iPhone only)