Compression leaking to valvetrain

OP, is the leakage coming up through the head by the pushrods? (That's what it looks like in the video.) If so, the compression is going into the crankcase (which this sounds like) and I agree with RRR... a ring/ringset is broken or ring lands busted out. And looking closely at the video, this pulsing of vapor does not maintain sync with #5 cylinder throughout the video, and so IMHO it can be coming from any cylinder.

Too late to run a compression test, which would have told us a lot; still could do the other side as suggested. IMHO, do that and get ready to pull the other head and look.

Was this a fresh re-bore or were the bores just honed? Broken rings/lands could happen due to detonation. Any hint of detonation sounds before this?