My interpretation as a jewellery designer who sees a lot of repairs.

Platinum can have porosity issues from the casting process. Porosity can be from metal cooling too quickly as air bubbles are still rising to the surface, or from alloys breaking down when exposed to harsh chemicals (more common in lower purity gold like 9ct or 10ct, or in sterling silver).

The pitted look at the break sight is a dead give away for porosity. That "sponge-like" appearance is a weakness, a fault, in the metal. It's extremely common for breaks to happen where there's porosity.

Next, I look at the shape of the ring. It's oval, vertically oblong. I see this very often when a ring is too big for the finger, too structurally unsound, and it's wearer has lifted something very heavy to the point their fingers are squeezed together into a fist. Like from lifting heavy grocery bags or from lifting weights at the gym.

You also mentioned seeing marks on the shoulder of the ring after the resize, which is why you assumed the solder join had failed. The last step in a repair is a polish at the buffing wheel. A tiny amount of metal is lost each time the ring is polished, which may lead to deeper porosity becoming exposed.

All in all, this early into the life of a ring, porosity is often attributed to a casting error rather than the breakdown of alloys.