Can not manipulate data with transformations i.e. merge


I use the transformation merge to combine data from an influx source with the source sun&moon to make use of sunrise and sunset values:

I can not change any field of the sun&moon table i.e. with the fill function to eliminate the null values in a column.

Any idea what could be the reason for this behavior? Or what could be a workaround?

I checked the debug window and recognized that all the functions are executed internally before the merge transformation.

How can I use the transformations after the merge?