with contrast stretching I meant that ArcMap might assign different min/max values when displaying the individual (non-mosaiced) tiles. Please have a look here if this is the case:
If you are using L2A products, there is not much you can do about the lines after mosaicing, because they are already calibrated to bottom of atmosphere. Some discussions on this issue are here:
- Sentinel -2 Mosaicking Problem in data
- No histogram equalization/color balancing in SNAP3's mosaicing modules
- Normalization of Sentinel-2 pixel values
Maybe also this is interesting to you: Copernicus offers new mosaic service of ESA Sentinel-2 L2A