Kindle device not showing in Amazon account randomly?

Hey all, I'm lost on what is going on and hoping someone can help.

Talk to me like your Gran because I am the opposite of a tech-head.

I got the Kindle basic (or matcha, or basic in matcha colour? Whichever) just before Christmas. I had an old Fire HD for years and the battery finally knackered too much to use for any length of time.

The new Kindle was registered to my Amazon, no issues. I could see it in my devices. I've bought and downloaded books without problems. I read in airplane mode to save battery. I have just visited the Amazon website and my Kindle is no longer showing in my devices, but the old Fire device (which I haven't deregistered) is still showing. I turned the internet on my kindle, checked the Kindle store and settings, all is well according to the device. I restarted it to see if it would appear, but it still isn't showing in my devices.

I can download books from unlimited. I've just bought a random £1 kindle book via the device to check and it has gone through fine. If I go into settings, it shows the option to deregister. The device thinks it is registered, but Amazon thinks it isn't??? What is going on? If I deregister and register again, will I lose anything currently on the Kindle? Will that likely fix the issue?

Thanks!