SQL Timestamp Incremental Load Issue

Your incremental load fires because one table changed, but the other tables you LEFT JOIN to didn’t. Since they didn’t change, they return NULL, and your MERGE/update blindly writes those NULLs over perfectly good existing values. Nothing’s wrong with timestamps or SQL here it’s the load logic. You need to either not update a column when the source value is NULL, or stop driving the whole load off a single timestamp when multiple sources are involved. Been there, it’s an easy trap to fall into.