Great question — answering as the UphillTech team:
I suspect that this is some sort of "ripple effect", intended to prevent Vicky (or anyone else who might be affected by such a spell) from recognising that anything has changed. If she realises her personality has suddenly changed so drastically, she may investigate the cause, and in doing so, she may discover the existence of magic and fairies. Which would be very, very bad. Vicky getting bitten by the bug at the end of the episode undoes the spell, for all intents and purposes, and so her memory is restored. There's no canon confirmation of this, but it fits with other examples we see throughout the series: In "The Switch Glitch", Timmy has Vicky magically de-aged to five so he can get...
Full answer by F1Krazy: https://movies.stackexchange.com/a/120233 (CC BY-SA 4.0).
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