  We characterise the planet-occulted line distortions (POLDs) in absorption
spectra of transiting planets, that arise from proxies used for the occulted
stellar lines and investigate the impact of stellar rotation, centre-to-limb
variations, and broadband limb-darkening. We used the EVaporating Exoplanets
(EVE) code to generate realistic stellar spectra during the transit of
exoplanets, accounting for the 3D geometry of the system's architecture and
atmospheric transit, as well as for spectral variations over the stellar disc.
The absorption spectra were calculated using approaches drawn from the
literature and compared to the expected signal. The POLDs from stellar rotation
are dominant for moderate to fast rotating stars, reaching amplitudes
comparable to atmospheric signals, but they can be mitigated by shifting the
stellar line proxies to the radial velocity of the planet-occulted region.
Centre-to-limb variations become dominant for slow rotators and are more easily
mitigated at the stellar limb. We re-interpret the ESPRESSO data of two iconic
systems and confirm that the sodium signature from HD209458b mainly arises from
POLDs. However, we unveil a possible contribution from the planetary atmosphere
that warrants further observations. For MASCARA-1b, we did not find evidence
for atmospheric sodium absorption and we can fully explain the observed
signature by a POLD for super-solar stellar sodium abundance. We studied POLDs
dependency on star and planet properties, and on the proxy used for
planet-occulted lines. Distinguishing planetary absorption signatures from
POLDs is challenging without access to accurate estimates of the local stellar
spectrum and system orbital parameters. We propose a way to mitigate POLDs and
improve atmospheric characterisation, by using simultaneous forward modelling
of both the star and the planet to simulate the global observed signatures.
