  We present a spectral study of the black hole candidate MAXI J1348-630 during
its 2019 outburst, based on monitoring observations with Insight-HXMT and
Swift. Throughout the outburst, the spectra are well fitted with power-law plus
disk-blackbody components. In the soft-intermediate and soft states, we
observed the canonical relation L ~ T_in^4 between disk luminosity L and peak
colour temperature T_in, with a constant inner radius R_in (traditionally
identified with the innermost stable circular orbit). At other stages of the
outburst cycle, the behaviour is more unusual, inconsistent with the canonical
outburst evolution of black hole transients. In particular, during the hard
rise, the apparent inner radius is smaller than in the soft state (and
increasing), and the peak colour temperature is higher (and decreasing). This
anomalous behaviour is found even when we model the spectra with
self-consistent Comptonization models, which take into account the
up-scattering of photons from the disk component into the power-law component.
To explain both those anomalous trends at the same time, we suggest that the
hardening factor for the inner disk emission was larger than the canonical
value of ~1.7 at the beginning of the outburst. A more physical trend of radii
and temperature evolution requires a hardening factor evolving from ~3.5 at the
beginning of the hard state to ~1.7 in the hard intermediate state. This could
be evidence that the inner disk was in the process of condensing from the hot,
optically thin medium and had not yet reached a sufficiently high optical depth
for its emission spectrum to be described by the standard optically-thick disk
solution.
