Rather than setting Trolls: The Beat Goes On up for a release date later in 2018, perhaps after a more traditional-for-Netflix 13-episode lineup was finalized, the producers and other execs decided that it would be more beneficial to give fans the earliest look possible by putting a smaller batch of episodes out as soon as they were completed.
Matt Beans’ answer is about the most fan-friendly and non-corporate-minded explanation possible.
That makes sense, too, as the Trolls spinoff series was only announced at the end of 2017, so it was pretty surprising to see it would already be premiering when it did.
Especially since the new holiday special aired not too long before that.