A workaround is turning off subpixel antialising.
I found similar artifacts in the font rendering in this issue.
https://github.com/electron/libchromiumcontent/issues/384
The libchromiumcontent issue links to this one, which describes why this happens:
https://bugs.chromium.org/p/skia/issues/detail?id=6663
With a recent change, FreeType no longer adds LCD padding by default, but only when this is necessary. This changes the bitmap offset and size.
Maybe a solution could be to update the bundled version of Chromium/NW.js?