Paint cracking at roof/quarter panel junction

So I restored my 1973 Mach 1 in 2020-2021, and the guys who did the paint did a really nice job. The roof panel and both quarters were replaced, and the joint was welded solid. The joint that had originally been filled with lead was filled with long-strand fiberglass filler that was covered with a layer of short-strand fiberglass filler and finally Rage Ultra body filler. The passenger side is perfect. The driver side started showing the outline of the joint late last fall, and now has progressed to cracking the paint.

I know that I will have to sand it down and grind it out to get to the root of the problem and correct it. My question is, after I repair the problem, and blend out the panel(s), will I have to blend the entire roof and both quarters together? I have heard that only clearing the repainted areas will allow the transitions to show up in some lighting conditions. If I have to scuff and reclear the roof and both quarters I will, I just want to confirm that that is what I'll have to do.

Thanks in advance.

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Paint cracking at roof/quarter panel junction