mergeSchema doesn't support all operations. In some cases .option("overwriteSchema", "true") is needed. MergeSchema doesn't support:

• Dropping a column
• Changing an existing column's data type (in place)
• Renaming column names that differ only by case (e.g., “Foo” and “foo”)

more on that topic here https://www.databricks.com/blog/2019/09/24/diving-into-delta-lake-schema-enforcement-evolution.html