That’s an issue with the GraphBuilder. Some operators don’t work well.
We have it on the development agenda: https://senbox.atlassian.net/browse/SNAP-1022?jql=text%20~%20"graph%20builder" But this might not be solved with the next release.
Workarounds have been discussed here:
SNAP 6.0 can't read bands by rayleigh correction in graph builder
and here:
Batch processing/graph file problem