File becoming corrupted.

That's disturbing and it shouldn't happen but you knew that already.

Unfortunately, I've seen this before. Do I need to replace my hard disk?

Not necessarily, but I recommend more than just one Time Machine backup, with at least one of them kept geographically separate from all others at all times. That means a minimum of three backup devices.

Then open Disk Utility and "verify" the disk containing those photographs. If it finds problems and it is the Mac's startup disk then "repairing" it requires booting macOS Recovery.

How to repair a Mac disk with Disk Utility - Apple Support

Usually, a traditional spinning hard disk drive exhibits symptoms of failure long before it fails completely. Those symptoms usually manifest in poor performance, giving you lots of advance warning before it becomes completely intolerable. Mysterious file corruption in the absence of other causes (see below) or symptoms is extremely rare.

Never use any non-Apple disk "optimizers" / "managers" / "cleaners" / "doctors" / "anti-virus" products / similarly categorized junk. Hopefully that just goes without saying.

Hard disks usually fail with more frequent read and write errors and bad sectors. I've seen none of this.

I agree.

It's really odd. What model Mac is it? For example certain portable Macs suffered from a peculiar SATA cable failure that was difficult to diagnose.

Well, SSDs certainly won't exhibit bad sectors and write errors before they fail. The bit rot you describe isn't going to get any better. I'd replace it.

👍

With only nine months on it, it should be a warranty repair.