A bit late to respond to this, but from my experience I had lines between merged images if I converted CRS of each image THEN merged. If I keep the CRS as is (usually UTM), merge then convert CRS that produced a seamless blend.
Also another point to add, usually images will have atmospheric correction (for Sentinel 2 for example: https://sentinel.esa.int/web/sentinel/user-guides/sentinel-2-msi/product-types/level-2a) in some cases,even after applying atmospheric correction, the images still wouldn't be 100% corrected, hence causing the issue above.