Good morning memememe,
yes that is quite curious.
The plant had a fungal infection in the apex area.
A few years ago I had mice in the greenhouse.
The mice also transmit fungi with their saliva when nibbling.
Here is a photo of another infected plant.
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And here the curious " cap " Astrophytum. . .
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This plant is probably beyond saving.
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Other plants have a different strategy for apex injuries (but this was not a fungus).
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Best wishes
K.W.