My friends say that the map cuts out when I give them the map scroll.
Instead of having the entire map discovered they get only one part of the map.
they say that their maps are not fully discovered even though I'm 100% sure I discovered the areas that they cannot see.
The same happened to me a few times when I was playing on public servers.
I'm pretty sure that to reproduce this glitch all you need is to have a world with a lot of data (for example a world older than 500 days ) and a fully discovered land.
Then just write a map scroll and give it to your friend and who does not have the map of discovered.