I am about to braze a boss onto a tube that is for practical purposes closed so I won’t be able to soak off the flux. Technically there is a path to if but it is though a 3mm hole, another tube and then another 3mm hole. If any flux ends up in the tube is it going to cause any issues? or should I try to tig the boss instead (much more messy visually)
Theoretically flux is a little bit on the acid side of PH and could cause a small issue but in actual use the answer is no.
I have never in all my years (30) seen it cause any issue whatsoever. Even during repairs to old bike frames there was only superficial corrosion due to flux. Framebuilders almost universally leave some behind and we have made millions/billions of bicycles this way without issue.