https://www.polygon.com/2018/11/28/18116050/pokemon-go-sinnoh-stone-research-rewards

Not long after dedicated Pokémon Go players received their first Sinnoh Stones, new reports suggest that Niantic has already changed the reward parameters. Instead of giving the special evolutionary item away as a guaranteed weekly prize, The Silph Road fan community is finding that Sinnoh Stones appear to be part of the random reward pool.

Sinnoh Stones came to Pokémon Go earlier this month, alongside an event encouraging players to hatch as many Eggs as possible. The Stones are the only way for players to evolve certain Pokémon into their newly added fourth-generation forms. When the company announced that it would introduce these special items into the game, it also detailed how players would be able to collect it: as an award for those who completed a full seven days’ worth of daily research tasks. (Polygon news editor and resident Pokémon Go phenom Michael McWhertor received a Sinnoh Stone on Monday as a result of his weeklong research breakthrough accomplishment, for example.)