Probably outdated spectral response functions of Sentinel-2A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.