So finally had a chance to redo my test with tail fuze plugged, and SOB if it doesn't work. 100% boom rate. Question still stands why equipping tail fuze causes such inconsistency, maybe that's the way its 'supposed' to work? Be nice to get some clarification on that, but for now I'll accept DON'T USE BOTH NOSE AND TAIL FUZE as the solution. Updated track for proof
spittybombs broken questionmark TAIL PLUGGED.trk
grifnizzle's post in Spitfire MkIV bombs (250 & 500) very high rate of failure (no boom) was marked as the answer