  Fano resonances are ubiquitous phenomena appearing in many fields of physics,
e.g. atomic or molecular photoionization, or electron transport in quantum
dots. Recently, attosecond interferometric techniques have been used to measure
the amplitude and phase of photoelectron wavepackets close to Fano resonances
in argon and helium, allowing for the retrieval of the temporal dynamics of the
photoionization process. In this work, we study the photoionization of argon
atoms close to the $3s^13p^64p$ autoionizing state using an interferometric
technique with high spectral resolution. The phase shows a monotonic $2{\pi}$
increase across the resonance or a sigmo\"idal less than ${\pi}$ variation
depending on experimental conditions, e.g. the probe laser bandwidth. Using
three different, state-of-the-art calculations, we show that the measured phase
is influenced by the interaction between final states reached by two-photon
transitions.
