Have you tried resetting your cache or re-launching your browser?
Because it seems like it’s not an Android issue based on the other Android users posting their screenshots.
The “jump to new” button only appears in a thread with new posts you haven’t seen, and it goes away once you’ve read the new posts.
Yes have cleared the cache, checked the updates on the make sure they are the most current version, cleared the phone memory and rebooted the phone.
All the things that a person checks to make sure their phone is working properly and up to date, then a person can move on to diagnosing other things as to why issues are happening or not happening.