Manuscripts move from "Under review" to "Required Reviews Complete" when all the necessary reviews to make a decision have arrived. This is usually journal-dependent, e.g. some journals can move the status when one review has been submitted, while others might ask for 2 or 3 or more. Manuscripts move from "Required Reviews Complete" to "Decision in Progress" when the editor's ready to make a decision.
In this case you didn't see "Required Reviews Complete", which could mean:
- Editor decided to make a decision before all the reviews have arrived. This could be for all sorts of reasons, e.g. the status moves when 2 reviews have arrived, but only one reviewer submitted a review and the editor's making a decision anyway. Or the editor decided to terminate the review process on the strength of that one review. Etc.
- Editor "got to" the manuscript before you did. In other words, there really was a "required reviews complete" status, but the editor was monitoring the journal at the time and moved the manuscript along before you saw the status change.
In all cases there's nothing to do except wait.
an intermediate step wasn't followed