If the radiator cap upper seal is proper and there is no obstruction from radiator neck to overflow, then there must be a sealing issue between the upper radiator lip and the cap upper seal.
There appears to be some sort of white residue at the radiator neck overflow entry/exit hole down low just below the overflow hose attachment point on the filler neck. Definitely check that there is not something blocking that passage in the radiator neck, the overflow hose, and the overflow tank passage.
The feature atoman pointed out is definitely worth investigating; however, I'd also be taking a closer look at the radiator lip in the bottom left (almost 180 degrees opposed to what atoman pointed out) as there is obvious rust showing there whereas you don't see that anywhere else on the radiator lip. Based on the rust, I would think this might also possibly be the area where coolant is escaping.
If you cannot find a blockage and atoman's finding doesn't turn out to be anything of concern, then I'd highly recommend inspecting that lower left area of the radiator sealing lip to see if it's not slightly bent (not perfectly level at this point) and, thus, possibly allowing coolant to escape there.
Had there been a bad radiator cap on there long enough with coolant leaking through this area over a long period of time, that flow may have eventually "worn down" and or eroded metal in the flow path, creating a shallow valley that now has problems sealing. That said, I don't see an obvious wear down area in your pictures; however, I've seen some pretty dramatic "rip tide" coolant bypass troughs worn into radiator sealing lips in the past... though more often down in the lower seal lip than the upper. Barring something like that, it also might just be a manufacturing imperfection in that radiator lip and or mechanical bending that has somehow occurred in that lower left area.
If everything is sealed and flowing properly, any problem creating excess pressure in the system should be blowing out furiously through the overflow instead of or in addition to at the cap. Lack of evidence of flow to/from the overflow says there is a blockage or very poor sealing of the radiator cap to radiator interface.
There appears to be some sort of white residue at the radiator neck overflow entry/exit hole down low just below the overflow hose attachment point on the filler neck. Definitely check that there is not something blocking that passage in the radiator neck, the overflow hose, and the overflow tank passage.
The feature atoman pointed out is definitely worth investigating; however, I'd also be taking a closer look at the radiator lip in the bottom left (almost 180 degrees opposed to what atoman pointed out) as there is obvious rust showing there whereas you don't see that anywhere else on the radiator lip. Based on the rust, I would think this might also possibly be the area where coolant is escaping.
If you cannot find a blockage and atoman's finding doesn't turn out to be anything of concern, then I'd highly recommend inspecting that lower left area of the radiator sealing lip to see if it's not slightly bent (not perfectly level at this point) and, thus, possibly allowing coolant to escape there.
Had there been a bad radiator cap on there long enough with coolant leaking through this area over a long period of time, that flow may have eventually "worn down" and or eroded metal in the flow path, creating a shallow valley that now has problems sealing. That said, I don't see an obvious wear down area in your pictures; however, I've seen some pretty dramatic "rip tide" coolant bypass troughs worn into radiator sealing lips in the past... though more often down in the lower seal lip than the upper. Barring something like that, it also might just be a manufacturing imperfection in that radiator lip and or mechanical bending that has somehow occurred in that lower left area.
If everything is sealed and flowing properly, any problem creating excess pressure in the system should be blowing out furiously through the overflow instead of or in addition to at the cap. Lack of evidence of flow to/from the overflow says there is a blockage or very poor sealing of the radiator cap to radiator interface.
