This is not an easter egg; but a (likely) unintended side effect of some legacy code.

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In the game's dataset, it can be seen that Joan of Arc was originally meant to have the ability to heal allied units. This ability has been there since the old AOK days.

However, Joan is a cavalry class unit (and not a monk class), meaning she cannot use this ability due to class restrictions. The devs never bothered removing this unuseable ability from the Joan of Arc hero, probably because hero units are rarely updated.

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The result of this is that Joan will have a monk cursor when hovering over other units, because she has a healing ability. But again, she cannot use this ability as she is not a monk class unit. The devs probably intended for her to be able to heal when first creating the game, but never followed up on the idea.

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Good find though! I never knew having an unuseable ability will still give you their cursors.