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Stellar spectroscopy provides useful information on the physical propertiesof stars such as effective temperature, metallicity and surface gravity (logg). However, those photospheric characteristics are often hampered bysystematic uncertainties. The joint spectro-seismo project (APOKASC) of fieldred giants has revealed a puzzling offset between the log g determinedspectroscopically and those determined using asteroseismology, which is largelydependent on the stellar evolutionary status. Therefore, in this letter, we aimto shed light on the spectroscopic source of the offset using the APOKASCsample. We analyse the log g discrepancy as a function of stellar mass andevolutionary status and discuss the impact of He and carbon isotopic ratio.  We first show that for stars at the bottom of the red giant branch, thediscrepancy between spectroscopic and asteroseismic log g depends on stellarmass. This indicates that the discrepancy is related to CN cycling. Wedemonstrate that the C isotopic ratio ($\rm ^{12}C/^{13}C$) has the largestimpact on the stellar spectrum. We find that this log g discrepancy shows asimilar trend in mass as the $\rm ^{12}C/^{13}C$ ratios expected by stellarevolution theory. Although we do not detect a direct signature of $\rm ^{13}C$,the data suggests that the discrepancy is tightly correlated to the productionof $\rm ^{13}C$. Moreover, by running a data-driven algorithm (the Cannon) on asynthetic grid trained on the APOGEE data, we quantitatively evaluate theimpact of various $\rm ^{12}C/^{13}C$ ratios. While we have demonstrated that$\rm ^{13}C$ impacts all parameters, the size of the impact is smaller than theobserved offset in log g. If further tests confirm that $\rm ^{13}C$ is not themain element responsible of the log g problem, the number of spectroscopiceffects remaining to be investigated is now relatively limited. [Abridged]

The spectroscopic indistinguishability of red giant branch and red clump stars