Hello there, I just joined because of the discussion people were having about my livestream from last Friday, as well as the article of the issue in question, which I just discovered. To go over a bit of backstory; some people over at the Sonic Mania Modding Discord server had found out about the playable Angel Island Zone. Not sure if it were fake or not, I've decided to stream the game using the 1.04 update. To everyone's surprise, it turned out to be real! After a bit of exploring and running around, we stumbled across the capsule where Mighty and Ray were trapped in, but since it has been confirmed that the version I played was actually an earlier version of Encore mode (suspecting that once the actual DLC is available and downloaded, the data from that will overwrite the current one that we discovered), they were both Sonic's. afterwards, we went into another cutscene which I skipped after going through it a second time, which suprisingly brought us to Encore Green Hill Zone. From there, I must've played through it until Lava Reef Zone act 2, where I was forced to shut the whole thing down by SEGA themselves, afterwards thanking me for responding to their reply. I was honestly very shocked to see a majority of Encore's content could've been enabled by using a backdoor of sorts.
From what I've experienced, Encore mode does not include any new achievements as of right now. It could always change though.