Do you mean the top fin touching the top plate or the end plate? What issues could this cause? Just wondering.
The core plate (end plate) which is where the tank attaches. The flux can get drawn away from the tube joints, and cause a leak. The cores, are assembled from the individual pieces of aluminum, and then they get sprayed with a powder flux, before they go into a brazing furnace. There needs to be flux at all the joints where each piece attaches to each other, for it to become one piece inside the furnace. I do not know the chemistry/physics of the actual brazing process, but I believe the flux just facilitates the melting of the aluminum pieces together. The furnace has a very low oxygen atmosphere inside. Nitrogen is used to displace the air.
The crusty white stuff you might see in spots is extra flux.
Edit: I'm sorry, if I've caused a mild panic...your radiator is probably fine, I was assume that it was leak tested when it was built, and passed. As I probably mentioned, I built these things in the past, and this is some of what I looked for, which was prior to a leak test.