Pending comments no longer visible to comment author when awaiting moderation

My blog, which I don’t wish to identify, does not allow users to register but allows readers to submit comments to blog posts by providing their name and email address.

I am the only admin and I have it set so that every single reader comment is held for moderation and must be approved by me before going live.

Before updating to 4.9, readers who had typed their comment and pressed ‘Submit’ could see their comment on their screen at the bottom of the blog post, with a message above it that I configured through my theme’s functions.php that said “Your comment is pending moderation”.

This was good because only the commenter could see this, and it let them know the site had received it without a problem.

But now I have been alerted that whenever anybody submits a comment, when the page re-loads after pressing submit, their pending comment does not show and they get no indication it has been successfully received, even though it has been.

This is bad because they quite fairly conclude that their comment is lost!

I have disabled my Captcha plugin and posted a test comment in incognoto mode but the problem persists.

I imagine 4.9 changes some references or something that means the references in my theme’s comments.php or functions.php are incompatible.

I can’t find any other report of this issue on the forum. I am quite knowledgeable and think I can sort it out myself if given the right steer!

Does anybody know what the problem could be, and/or what I can do to fix it?

Any help would be appreciated.