This issue comes and goes, it sometimes happens, and sometimes doesn't. Also with books and it's even more frustrating with them. (And it likely doesn't depend on what text you input)
Ok, there's a lot of wrong ideas in here:
1. Nintendo don't let you use SD Cards for savedata so please stop from saying that you have a lot of space in your SD.
2. Nintendo haven't implemented yet an external data metod so games can't save any data outside the save data file.
3. Nintendo have restricted all save data sizes to 2GB (with exceptions in some games like Dragon Quest Builders 2) because they have cloud service to store a copy of your saves and Minecraft is compatible with it.
Edit:
Hey, one moment. What's the maximum size of shared data? That file that is used by games like 1-2 Switch or Animal Crossing New Horizons.
I'm not english, sorry if I make any mistake in my comment.
Ewan Maclnnis:
No, in Nintendo Switch games can extend their savedatas. If you run Minecraft the first time, savedata is like 500MB but it can reach 2GB maximum. And thats where the problem is. I don't know if you need Nintendo permissions or it is just in Minecraft code but Mojang needs to increase savedata lenght.
Maks:
Savedata can only be saved in Memory. You can't use SDCards to store they. ¿Do you have free space in system memory?
Still affects 1.16.1 Hotfix. (I'm in Nintendo Switch)
It happens to me too. I don't think it's because save data is restricted to 2GB because my Dragon Quest Builders 2 savedata has a lenght of 3.7GB.
You have Minecraft Bedrock Edition. Go to -> https://bugs.mojang.com/projects/MCPE/summary
Try to make one advancement. They may be hidden.
The signs prior to 17.3.x used to work just fine, so it has to do with this newer Realms' version. (Except for line breaks, that issue isn't new)