Probably outdated spectral response functions of Sentinel-2A
Claverie, M., Ju, J., Masek, J.G., Dungan, J.L., Vermote, E.F., Roger, J.-C., Skakun, S. V., Justice, C., 2018. The Harmonized Landsat and Sentinel-2 surface reflectance data set. Remote Sens. Environ. 219, 145–161. https://doi.org/10.1016/J.RSE.2018.09.002:
Note that the HLS v1.3 dataset discussed here relies on the MSI Relative Spectral Response (RSR) v2.0. ESA subsequently issued corrections to the RSR functions for bands 1 and 2 for Sentinel-2a. HLS version 1.4, slated for release in 2018, will update the RSR functions for these bands.
So, probably we need to update the response functions for S2A.
Edited by Daniel Scheffler