The box looks like to be XORed, probably forgot to reset composition mode state after rendering the brush outline.
Git commit b6e9d537070af23b211eaf8ef5ff1faa2f36d83f by Alvin Wong. Committed on 17/09/2017 at 16:09. Pushed by alvinwong into branch 'krita/3.3'. Fix paint tool colour picker display witn QPainter canvas M +2 -0 libs/ui/tool/kis_tool.cc https://commits.kde.org/krita/b6e9d537070af23b211eaf8ef5ff1faa2f36d83f
Git commit b1f67e34fce0b6ada0e9c06d9feab8c765891677 by Alvin Wong. Committed on 17/09/2017 at 16:10. Pushed by alvinwong into branch 'master'. Fix paint tool colour picker display witn QPainter canvas M +2 -0 libs/ui/tool/kis_tool.cc https://commits.kde.org/krita/b1f67e34fce0b6ada0e9c06d9feab8c765891677