This happens for me with an older Firefox on an older Mac. You didn't state your browser, os, or platform versions, so comparisons may be inexact.
I see this happen regularly when there are images in posts. The images start out unloaded, and so take no space. Then when they load, it changes the page layout and scrolls the post I wanted to see completely out of view.
All it takes is one image in a post above the target post on the page. It's much worse with multiple images, because each one increases the "navigational error", until eventually, even though you started out with a heading for San Diego bay, you end up somewhere north of San Francisco bay.
In threads without images, it works fine for me. I don't recall ever seeing a problem. Try it with some threads here in Site & Forum Feedback.