I always interpreted it as Jake ordering the erasure of too much memory. His passive powers seem to operate by burying a person's thoughts until they're just subconscious or rewriting the memory, but his active command seem to outright force someone's brain to delete it. Jess didn't react that way when ordered to forget something but it would likely only be getting rid of minutes for her. But Mia would have every stray moment where she thought about Ella's existence and Jake's powers purged from conscious and subconscious memory across days.
Considering the brain technically permanently reshapes itself every time it stores something in memory, with long term memory requiring bigger and more structurally significant changes, I can only imagine the kind of strain having a big chunk of your synaptic connections forced to break up and warp beyond all capacity by a supernatural foreign power would bring onto someone. It's not a slight twist or shuffle like misremembering something, small to medium subtle corruptions of old memories happen without you noticing everyday. Maybe you were fatter back then, maybe that shirt's the wrong shade of green, and in your head a shout or a whisper is equivalently loud. But to outright purge everything, wipe the slate clean and force the pieces to never be allowed to be whole again? I'd imagine it'd just be too much happening too fast.
Considering the brain technically permanently reshapes itself every time it stores something in memory, with long term memory requiring bigger and more structurally significant changes, I can only imagine the kind of strain having a big chunk of your synaptic connections forced to break up and warp beyond all capacity by a supernatural foreign power would bring onto someone. It's not a slight twist or shuffle like misremembering something, small to medium subtle corruptions of old memories happen without you noticing everyday. Maybe you were fatter back then, maybe that shirt's the wrong shade of green, and in your head a shout or a whisper is equivalently loud. But to outright purge everything, wipe the slate clean and force the pieces to never be allowed to be whole again? I'd imagine it'd just be too much happening too fast.