If any portion of a layer goes outside of the canvas edges, that portion will not appear in the G'MIC preview and will not be affected by any filters. One fairly impractical workaround is to use "Select" -> "Select All", and then "Select" -> "Grow Selection" with a big enough value to select the full layer contents (being able to do this may be a bug in itself, see my comment on https://bugs.kde.org/show_bug.cgi?id=380343 ).
The same goes for all filters, so I'm not sure that this is a bug. We disabled filtering of layer data outside the canvas boundaries because people got really surprised when they accidentally created enormous layers this way and didn't understand why the filters were taking so long. I'll close this for now until we're ready to revisit the question of whether layer data outside the image should be modified by user actions.