Leaves become translucent, soften and dissolve, often within days of planting.
Short answer
Most melting in new plants is conversion from emersed to submersed growth, not death. Leave the roots alone, remove the dissolving leaves, and wait for new growth.
Ordered by urgency, then by how commonly each one turns out to be the answer. The discriminator is the observation that separates this cause from the others — use it before acting.
How to tell: Anubias, Java fern or Bucephalandra planted with the rhizome under the substrate; the rhizome softens and blackens.
How to tell: Gradual decline over weeks rather than sudden melt, often with yellowing or holes before dissolution.
How to tell: An established plant melts after a change — new CO₂, a lighting change, a large water change with very different water.
How to tell: Newly planted, and the species is normally grown emersed. Old leaves dissolve while the crown and roots stay firm.