Just want to say thank you for all your views. Took it down to the shop and they said it was cosmetic - but they have sent the pictures to Brompton who will respond in a day or so. Hope to receive something positive from Brompton (not doubting the integrity of the bike but was somewhat disappointed with it).
My understanding is that Brompton uses a machine to do all the brazing now with computer-controlled gas jets and heat sensors. Probably they missed a little cleanup work as above. I wouldn't worry about the strength of the joint.
As someone who has worked with iron and metal my entire life, including welding and inspecting welds. I can tell you that the answer is yes and no.
Yes the braze looks fine. But,... no it wasn't cleaned up before the finish was applied. The slag should have been chipped off, and wire brushed, then the area would next be buffed of burn marks, scratches and weld splatter.
Someone cleaned up all the welds on the frame but missed that one, maybe he was distracted by someone or something, or it was Monday morning and he wasn't paying attention. Somehow it also got past inspection (or nobody noticed) before it went to the paint booth. Then on top of that it got all the way down the assembly line without anyone noticing or caring. Then it got past the final inspection and boxed. This doesn't look good for Brompton's quality control, makes it looks like quality is going downhill.
it's purely cosmetic but if i was going to pay a premium for this raw lacquer and almost $1700USD plus taxes for a typical 3 speed brompton I would expect it to be cleaned up without such glaring flaws. In a walmart bike i would turn a blind eye but not when they are running up to the range close to $2k. if you buy any brand new $1000+ bike you would expect the quality and finish of the frame to be near perfect.