Q: What does it mean if I receive an acceptance letter but the status remains 'Under Review'?

This is very unusual. So, we understand you mean an ‘acceptance letter’ and not an ‘acknowledgment letter.’ This is because you haven’t mentioned whether there were any updates after the status of ‘Submission Being Processed.’ Because after that, if the editor does decide to send the manuscript for peer review, the status would have first changed to something like ‘Reviewers Assigned’ and then to ‘Under Review.’ After the review got over, it would have changed to something like ‘Reviews Completed.’ It’s rare for a manuscript to directly go to acceptance (except in the case of a predatory journal, and that is not the case here as Brill is a reputed publisher with a long history), unless of course, the quality was right on the mark. :) If so, great! However, you should check this discrepancy, because (even) if accepted, the manuscript will still have to be proofed before being published, and the status(es) would need to change to reflect all these milestones.

For help with writing to the journal, you may find this handbook useful: A practical handbook of templates for communicating with the journal

And for a better view of journal status updates, you may find this handbook useful: A guide to understanding journal statuses

Both handbooks are available from R Upskill, a sister brand providing learning programs for various aspects of a researcher’s work, including writing, submission, publication, and research promotion. What’s more, the programs/courses are available for free for a limited period, so do visit the platform to know more.

For now though, you should write to the journal. And if it indeed is an accept decision, congrats for it!