Unpacking (SLA) articles - Episode 7: "The effects of explicit instruction on written accent mark usage in Spanish heritage language courses"

My own summary and views on a study by Antoni Fernández Parera & Andrew Lynch (2021): "The effects of explicit instruction on written accent mark usage in basic and intermediate Spanish heritage language courses." Journal of Spanish Language Teaching, 8 (1), 16-31. How helpful spelling instruction is. Does teaching accent rules explicitly lead to an increase in accuracy?  Does it have the effect we think it does, or does it have any unwanted side effects?