[OVT]

That is N. stellata.
The dying leaf is the outside leaf = the oldest. Rather common with these plants.

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[OVT]

I disagree as some "melting" leaves in Nymphaea are rather common: 1-2 melting leaves surrounded by 20+ perfectly healthy ones. The ones that melt are either the oldest leaves on the plant or the youngest.

You can also achieve the same result by:

• slightly "folding" the stem
• trimming the roots
• too long exposure to air
• temperature change up or down
• replanting