What was the "optical illusion" that led to erroneous metal concentrations in stellar atmospheres in the galactic center?
The journal paper is Thorsbro et al. (2018).

The facts are somewhat mundane. The atmospheres of cool M-giants are not well understood in detail. The infrared lines of neutral Scandium that had previously been used to claim massive overabundances in stars near the Galactic centre are problematic.

The paper by Thorsbro et al. shows that the same "anomalies" are present in other M giants nowhere near the Galactic centre. Instead they argue that the details of line formation, especially the formation low excitation lines formed high in the atmosphere, like the IR transitions of neutral Scandium, are particularly vulnerable to poorly modelled NLTE (non local thermodynamic equilibrium) effects. If analysed with atmospheric codes that assume LTE, then you get the "illusion" of a much higher abundance.