This sounds as though it could be a problem with the Time Machine browsing interface rather than with the backups themselves, particularly since your backups appear to be running normally and you can still access older Finder windows.
Before attempting anything drastic, I would suggest checking which backups macOS can actually see.
Open Applications >> Utilities >> Terminal, type tmutil listbackups, and press Return.
Compare the dates listed with those visible on your external drive and those available in the Time Machine browser.
If the newer backups appear in Terminal but are missing from the graphical interface, that suggests macOS recognizes them and the problem may be related to browsing.
If only older backups appear, the newer backups may not be registered or available through the normal Time Machine backup system, even if you can see files on the external drive.
If you receive an error or no results, check that the external drive is connected and mounted before trying again. Bear in mind that this command lists backups recognized by Time Machine, not necessarily every dated folder or snapshot visible on the physical drive.
You could also try restarting your iMac, reconnecting the external drive, and entering Time Machine again. Apple's guide explains how to browse and navigate through Time Machine backups, including using the timeline and arrows: Restore items backed up with Time Machine on Mac - Apple Support (CA)
It would also be worth opening a Finder window for a folder you know has changed recently, entering Time Machine, and checking whether you can retrieve an earlier version of a file. If you can restore an older version, that is a useful indication that at least some of your backup data remains accessible, even if the navigation controls aren't behaving as expected.
Apple also provides troubleshooting guidance for situations where Time Machine cannot use an existing backup: If you need to create a new backup from Mac - Apple Support (CA).
I would avoid erasing the external drive, deleting snapshots, or starting a new backup set until you've established that your existing backups are safe and accessible.
Since you are ‘running macOS Tahoe 26.7.1 on an Intel iMac, check System Settings for any applicable macOS updates. If the problem started after a system update, that detail could be useful when investigating a possible software bug.
One question that might help narrow this down is whether the backups after the end of August are completely absent from the timeline, or whether you can see them but clicking the navigation arrows takes you back to December 2024.
If tmutil listbackups shows the newer dates, that would be a useful clue to include when contacting Apple Support.
It would not conclusively prove an interface bug, but it would help distinguish a browsing issue from a problem recognizing the backups themselves.
