The increased integration of photovoltaic (PV) power plants operating in grid-following (GFL) mode has raised concerns about small-signal stability. The grid-forming (GFM) mode appears to be a plausible solution to these issues, generating interest in studying the small-signal stability of power systems dominated by power electronic converters, such as those in PV plants. However, stability studies generally neglect the dynamic phenomena related to the DC-side, specifically the DC-DC converter and the PV, which can lead to inaccuracies. This work investigates the impact of modeling the DC-side of a PV power plant operating in GFM (PV-GFM) on the small-signal stability of a power system. Modal analysis is performed on a test system under various operating conditions, including PV-GFM plants and converter-interfaced generation (CIG) in GFM where the DC-side is neglected. The results show that oscillatory instabilities occur when the DC-side is modeled. On the other hand, the system performs better dynamically when CIG-GFM is considered. Additionally, the oscillatory characteristics of the modes change, and new systemic interactions appear. The findings demonstrate the significant impact of the DC-side dynamics of PV-GFM on small-signal stability and highlight the need for accurate representation.