We can now put that rumor to test. AMD Technical Marketing guy Robert Hallock confirmed on Reddit that 2nd gen Ryzen processors will use solder. This is preferable as it results in lower temperatures, and higher overclocking potential. It's more expensive though, so that's probably the reason why AMD is using thermal compound for the Raven Ridge parts.
AMD is expected to introduce the 2nd gen Ryzen processors in March 2018. These will be manufactured using the GlobalFoundries 12nm node.