  It is commonly assumed that the properties and geometry of the accretion flow
in transient low-mass X-ray binaries (LMXBs) significantly change when the
X-ray luminosity decays below $\sim 10^{-2}$ of the Eddington limit ($L_{\rm
Edd}$). However, there are few observational cases where the evolution of the
accretion flow is tracked in a single X-ray binary over a wide dynamic range.
In this work, we use NuSTAR and NICER observations obtained during the 2018
accretion outburst of the neutron star LMXB 4U 1608-52, to study changes in the
reflection spectrum. We find that the broad Fe-K$\alpha$ line and Compton hump,
clearly seen during the peak of the outburst when the X-ray luminosity is $\sim
10^{37}$ erg/s ($\sim 0.05$ $L_{\rm Edd}$), disappear during the decay of the
outburst when the source luminosity drops to $\sim 4.5 \times 10^{35}$ erg/s
($\sim 0.002$ $L_{\rm Edd}$). We show that this non-detection of the reflection
features cannot be explained by the lower signal-to-noise at lower flux, but is
instead caused by physical changes in the accretion flow. Simulating synthetic
NuSTAR observations on a grid of inner disk radius, disk ionisation, and
reflection fraction, we find that the disappearance of the reflection features
can be explained by either increased disk ionisation ($\log \xi \geq 4.1$) or a
much decreased reflection fraction. A changing disk truncation alone, however,
cannot account for the lack of reprocessed Fe-K$\alpha$ emission. The required
increase in ionisation parameter could occur if the inner accretion flow
evaporates from a thin disk into a geometrically thicker flow, such as the
commonly assumed formation of an radiatively inefficient accretion flow at
lower mass accretion rates.
