If combustion chamber gasses are leaking into the water jacket, that would cause an overflow? But, could it be the radiator cap, or rubber seal on the cap?
On mine, I noticed a couple of things. The new 'OEM' cap that I installed with the CFS radiator was making quite a bit of rust at the valve seal. However, the truck came with a cap that looked original, based on the Japanese writing on the top, but has absolutely no rust. Also, the valve seal at the radiator cap conforms (makes an impression on the seal) to the lip on the inside of the radiator, for both the rebuilt OEM radiator and it's respective cap, as well as the CFS radiator and new 'OEM' radiator cap. The issue I've had is there is a lack of precise concentricity where the cap was seating, so by merely taking it off and replacing it, you get the old impression on the seal that may not line up with the radiator-lip on the most recent installation. I replaced it with a new Motorad non-venting cap, and am going to treat the radiator cas disposable, or one-use only. The Gates venting-type cap that I was using on my pickup wasn't rusting, and the two trucks use the same aftermarket caps, so that is how I got derailled into using aftermarket equipment.
I changed the coolant this spring; it was due after a couple of years. I used pre-mixed coolant. I have strong reservations against using tap-water in the cooling system; the stuff tastes like garbage, smells bad when you fill a 5-gallon bucket with it, and plants hate it. If I was doing a flush, I'd chase it down with a large volume of distilled water, likewise for mixing coolant, distilled-water, only. However, I can't find any information confirming the practice, but they do sell anti-scaling solutions.
Scale Formation - Penray - https://penray.com/resources/cooling-system-tech-facts/scale-formation/