= Solution
In an isothermal hydrostatic atmosphere, pressure falls as $P(z)=P_0e^{-z/H}$. If a water-band line core becomes optically thick at pressure $P_{\rm line}$ and an opaque cloud fixes the nearby continuum at $P_{\rm cl}$, the <exoplanet transmission spectrum> feature spans
$$
\frac{\Delta z}{H}=\log\frac{P_{\rm cl}}{P_{\rm line}}.
$$
A two-scale-height feature therefore requires
$$
\boxed{P_{\rm cl}=e^2P_{\rm line}}.
$$
Taking a representative near-infrared water-band pressure $P_{\rm line}\sim1\,{\rm mbar}$ gives $P_{\rm cl}\sim7\,{\rm mbar}$, so the appropriate estimate is an <exoplanet cloud deck> top near $10\,{\rm mbar}$. The numerical value scales directly with the assumed line-core pressure.
Other explanations include a high <mean molecular weight>, subsolar water abundance, a colder terminator, <atmospheric haze>, patchy two-limb clouds, stellar contamination, or instrumental systematics. Optical scattering slopes, broader <James Webb Space Telescope> molecular coverage, repeated transits, secondary-eclipse spectra, phase curves, and precise mass and radius measurements can distinguish these possibilities.
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