Low-mass stars experience a series of changes when they enter their post-main sequence evolution, including slower rotation velocities and deepening of the surface convective zone. This last process, the first dredge-up, dilutes the abundance of light elements. According to standard stellar evolution, which considers convection as its only mechanism of mixing chemical species in stellar interiors, low-mass stars are not expected to have any changes in composition after the end of the first dredge-up. However, observations of RGB stars show that there is another episode of abundance changes after the RGB bump. The physical process behind this second phase of abundance changes is still not well understood.
This is in part because of the lack of characterization of this mixing and its dependence on stellar parameters, such as mass and metallicity. Large spectroscopic surveys such as GALAH or APOGEE can help, especially when combined with asteroseismology (either from Kepler, K2, or TESS), but dedicated observations of elements such as lithium or carbon isotope ratio can give additional clues.