One of the issues here is the fact that Pt/Co oxidizes. Most likely, when you soldered the two rings together, it took a little too long and the solder melted and attached to the oxide instead of the platinum. Then, after a little wear, it simply broke off.

As you know, 1300 Pt solder contains no Pt at all, but is a mixture of silver and palladium and copper. The cobalt will begin to oxidize at about 1000 C., so the 1330 C solder will be inside the oxidation range. For this reason, I recommend you use a hard WG solder, which melts below 900C and thus is outside the oxidation range of cobalt. Also, in that case you would need flux (which you would NEVER use over 1000 C.