Q: Why did the status change back to "Under associate editor review" after being "Under review" for a few days?

From what I understand, your status was "Under associate editor review" for some time. Then it changed to "Under review" on Jan 12. This means that the paper cleared the initial editorial screening and was sent out for peer review. Now let's delve a little deeper and understand what was going on at the backend. Once the editor decided that your paper would be sent for peer review, he/she identified 2-3 reviewers and sent out review invitations. Once all the reviewers accepted the invitations, the status changed to "Under review."

However, after a few days, on Jan 18, the status changed back to "Under associate editor review." This was probably because one of the reviewers, who had initially accepted the invitation, declined it later. Possibly, as the reviewer started going through your paper, he/she discovered a conflict of interest. Another possibility could be that the reviewer felt that he/she did not have the necessary expertise on certain aspects of your paper and would not be able to do justice to it. What this change of status means is that the editor will now have to look for another reviewer. Once this reviewer accepts the review invitation, the status will change back to "Under review." Do not worry; this is a fairly common situation, and hopefully, will soon be sorted out.

Related reading:

• What does a status change from "under review" to "under editor evaluation" and back to "under review" mean?
• What does a status change from "Editor assigned" to "Under review" and back to "Editor assigned" mean?