It is a known "effect" with either that Tools -> GEGL Operation or a GEGL filter such as Gradient / Any of the filters with a 
against them in the menu.
When finished using the filter, change to a non-GEGL tool, I use the paint tool but you could use the measure tool ( for safety).
against them in the menu.When finished using the filter, change to a non-GEGL tool, I use the paint tool but you could use the measure tool ( for safety).