Deleting and inserting again the block does not help. I can see the problem even on flowchart created from scratch in GR 3.8 The problem exists on openSUSE Tumbleweed (GR 3.8.2), leap 15.2 (GR 3.8.1) and also in Windows port of 3.8.1 --Try deleting the block, saving, and adding the block back in. I triedsource -> log power fft -> max ... with no problems.
Wojciech Kazubski