This is rather unusual. Generally, once the status changed from "Under review" to "Decision in process," it means that the review is done and the editor can now make a decision on the paper. The status can change to "Under review" again if the editor is not satisfied with the reviews that have come in and sends the paper for an additional review. However, this cannot be done in one day as the editor will have to go through the reviews first, then find another reviewer, send out the review invitation, and receive an acceptance letter from the reviewer. Also, it's odd that the status has switched between "Under review" and "Decision in process" four times in a row. I feel this could indicate some problem with the submission system. You should write to the editor immediately explaining that you are concerned about this and ask what the current status actually is.
- What does a status change from "Required reviews completed" back to "Under review" mean?
- Why does the status change back to "Reviewers assigned" after 3 days of "Under review"?
- What does a change from "awaiting editorial decision" back to "under review" mean?