  Radial metallicity trends provide a key indicator of physical processes such
as star formation and radial gas migration within a galaxy. Large IFU surveys
allow for detailed studies of these radial variations, with recent observations
detecting central dips in the metallicity, which may trace the impact of
various evolutionary processes. However, the origin of these dips has not been
conclusively determined, with suggestions that they may be diagnostic
dependent. In this paper, we use the SDSS-IV MaNGA survey to investigate
whether the observed dips represent genuine decreases in the central
metallicity, or if they could be an artefact of the diagnostic used. Using a
sub-sample of 758 local star-forming galaxies at low inclinations, we
investigate in detail the impact of using different strong line diagnostics on
the shapes of the returned profiles, and the prevalence of dips. We find no
clear evidence of the dips being caused by changing values of the ionisation
parameter within galaxies. To investigate physical causes, we explore both
global and spatially-resolved parameters, finding that galaxies exhibiting
central dips in the O3N2 metallicity profile have on average lower H$\alpha$EW
values out to $R/R_\rm{e} \sim 1.5$, and higher values of D$_N$(4000) in the
central regions. We additionally find a higher prevalence of dips in galaxies
with high stellar mass, and lower values of global specific star formation
rate, suggesting a possible link to central quenching. Nevertheless, these
results are dependent on the diagnostic used, suggesting caution should be
taken when interpreting observed features in galaxy metallicity gradients.
