Here is some more information. I hope this makes sense, I tend to ramble on.
When welding aluminum, you really get very little weld penetration. If you were to cut the frame apart, you might see "drop-though" but very little penetration. Also, welding softens the material quite a bit. The more you weld, the softer it gets. If you do a weld repair, to gain original strength, you would need about 3X the amount of material (the heat zone always is much further along that the extra material) . Welds also shrink and as you weld pieces together other odd things happen like grain and metallurgical make-up alterations. WAIT! the tube is also made from an extrusion that has a longitudinal <may be wrong word> grain a bit like wood and has a tendency to split (only seven tubes are drawn, BB/headtube extruded)
When that frame was welded, it was made of hard pieces of metal and when welding the entire frame got quite hot. The areas that had thick pieces of metal (head tube, BB shell, drops) get so hot that when the welding is completed, it's as soft as the heel of a work shoe.
Think about this like cooking, we have now melted the butter, cinnamon and sugar together but we still don't have a cake.
Now, the frame goes in an oven for two heat cycles. The first cycle completes the heating of the entire frame that we started in welding (the entire frame is soft) then it is "baked" to hardness.
When a frame is cracked and you do a welding repair, the first thing I do is figure out what caused the failure. In this case, the head tube is too thin. The other contributing factors are the "interference" (press-fit) of the cup adding constant pressure, intermittent high loads (fella covered with mud, jumping) and the molecules of the tube were totally scrambled during welding.
We also have the fact that the strong down tube is holding that thin head tube quite tightly. This is like the belt loop on your trousers.
So we have all these things, these forces and circumstances with only a little ring of metal holding the bearings in place and "POP" it gets a crack.
So now, the crack has opened up and the bearing has a wee little bit of pressure, not the 5,000 PSI we once had (this is why hose clamps are silly for this issue). The cup only goes in 10mm so it can slosh around (consider the 1/2 meter-long cheater bar that looks like a fork)
So now we get it welded. We made the crack vanish but is it really gone?
No. What is there is a greater quantity of a very soft material. The head tube shrunk a bit during welding and when the cup was pressed in, it just got bigger to accommodate the bearing.
The area where the weld was done is like sponge, full of porosity due to contamination and extreme temperature and since it was the softest has literally turned to weak fabric and stretched without cracking (and will keep stretching). Strike it with a small hammer and punch. It's soft.
So what happens now? Well, the head tube won't break off. It will continue to stretch and allow the lower bearing to rock and eventually be damaged. I am sure you can go many more miles if required.
I personally would take a frame failure as an invitation to shop for a suitable upgrade..
