Update: the load time doesn't seem to depend on whether the save data is doing a background sync to Nintendo cloud. So isn't a file lock issue.
I wonder if it is something as simple as growing log files like Microsoft word 15 years ago.
I notice the "save data cloud" backup is running really slowly for the problem user account. I'm surprised it's backing up anything because I've just re-installed save data from Nintendo cloud to try to solve this issue!
Perhaps Mojang can allow us to save our worlds out somehow to give a work around for this mess. Either manually in the game ('coz Nintendo won't allow you in the file system) perhaps via transfer to another user account on the device. Or provide realm access as part on the Nintendo online subscription.
Has anyone tried transferring save data to another console? I suspect it'll work like the Nintendo cloud backup. Just wondering if anyone tried it?
I have had this issue Tried archiving the game, Didn't solve the problem. I thought it seems to be related to the new minecraft update, BUT as previously mentioned the aka.ms step appears to work (i did see the XBOX account name appear) but then fails after a short while. My theory was that the XBOX account wasn't playing nicely with the new minecraft code. So maybe refreshing / renewing the XBOX account might solve it.
So I updated my son's minecraft account via XBOX online webpages. I just changed the colour of the background, so a very simple change. Then the login is successful. I've not had a chance to see if auto-login after swapping to an alternate game still works though, so whether this is a 'real' fix.
My theory is that the new minecraft code needs a recently updated (or newly created) XBOX account to login successfully. Perhaps Mojang testers can look into the XBOX "account last update" timestamp as an approach to resolve this really irritating bug.
The switch 13.0 update has fixed my son's Minecraft load issue (was taking 45 mins to load). Now the game loads in 3-4 mins, logs into Microsoft, all his worlds are still in the list and I think extra packs etc. Data on the console now reads 756 Mb or so, but the switch online backup still says 2 Gb
I'll ask him to play for a bit, and then re-check the on console and switch online data sizes.
Thank you @Gavin Coffey for the update that there was movement on this issue.