I have installed fresh new Blufin GTS, set “strict” setting and excellent problem does NOT appears.
Upgraded to stable channel did the same test and problem also does NOT appear.
Holly caw, this looks like problem only on my original Blufin. Looks like I have messed something to get into this stealing focus window issue.
EDIT: I have now migrated to new fresh installed Blufin 40 and I have no window stealing focus issue anymore.