For example use this text:
This is A Test
This is B Test
This is C Test
This is A Test
This is B Test
This is C Test
This is A Test
This is B Test
This is C Test
This is A Test
Then filter with A. You will see:
This is A Test
This is A Test
This is A Test
This is A Test
Now, select all visible lines and apply lowercase. Remove filter. Lowercase has been applied to all the the text.
I lost 5 days of work due to this.