I've been a professional bronze/brass welder for 20 years. The weld looks fine. It's not visibly ideal, as it doesn't straddle the join equally, but we can only see the outside of the weld. There are techniques that are used to insure there's brass on the inside of the join as well, so what's visible is only half the story. The only "porosity" looks to be a bit of overcooking the brass where it was tacked up at the corner of the miter. Easy to end up with, since a good tack requires a lot more heat than laying down a fillet, and indicative of the techniques used to insure there's brass on the inside of the join.
The headstock uses much thicker tubing than the down tubes, so would require the angle of the torch very much biased toward the headstock to avoid the brass running up onto the downtube. Fillet brazing is slow-motion welding, so it ends up being very deliberate. If you're working quickly, laying down that stretch of fillet would take at least 30 seconds, so I'd have a hard time believing an expert such as Titmarsh, would end up with the fillet in the wrong place over that amount of time.
With Titmarsh's expertise and experience making these frames for years, I wouldn't think about it again and wring its neck without a worry. I've seen and fixed WAY "worse" than that on bikes that had been run for decades without an issue.
