What was the "optical illusion" that led to erroneous metal concentrations in stellar atmospheres in the galactic center? Articles summarizing a recent result about certain heavy element concentrations in stars near the galactic center all say that earlier reports about high levels in stellar atmospheres were the product of an "optical illusion". Examples include


UPI.com Study explains optical illusion at the center of the Milky Way
Science Daily Mystery at the center of the Milky Way solved
Phys.org High levels of scandium near the galaxy's giant black hole were illusory, astronomers find


At least Phys.org goes on to explain with real "science words":


  According to the new study, the lower temperatures of the giant stars helped to create the optical illusion that appeared in the measurements of spectral lines. Specifically, it means that the electrons in the elements behave differently at different temperatures, which in turn can be misleading when measuring the spectral lines of elements in different stars. The conclusion is the result of a close collaboration between astronomers and atomic physicists. (emphasis added)


Question: What was the "optical illusion" that led to erroneous metal concentrations in stellar atmospheres in the galactic center? Beyond just "electrons in the elements behave differently at different temperatures" (because how electrons behave in atoms at different temperatures should be at least fairly well understood by astronomers) what specifically is the source of the misunderstanding? Is it atomic physics, incorrect assumption just a mistake, something else?

The mention of "close collaboration between astronomers and atomic physicists" suggests that the answer will turn out to be fairly interesting!
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.