AMD may be experimenting with ways to separate processing cores, built on the latest tech they can get their hands on, and cache. The cache could be built using second best - now GlobalFoundries' 12mm, later something like TSMC 7nm. Static RAM doesn't scale well with node shrinks - at least the surface area doesn't scale well, I don't know about performance and power. So the cache is possibly a good candidate for being offloaded to a cheaper die, the latency would obviously go up but maintaining cache coherence would be an easier task, higher latency can also be mitigated with increased size, and AMD needs to keep buying something from GloFo anyway.