The new forum remembers the pages you have read. If, for example, a thread has 30 pages of whom 3 unread. Then you read 2 of them, and the forum still marks is as unread because you didn't read the 3rd page.
I hope this is he problem
"If you can't fix it with a hammer, you've got an electrical problem"
No, that's not the issue here. You have reached your maximum of unread posts, so the forum software marks the oldest of those posts as read. That's all.
Please take a breath and ask if you have any questions. Take it easy
