Downloading a world from realms to single player may change some data to incorrect ones.
I noticed it when I downloaded my world that I played with friends to single-player, we used a custom datapack to add advancements, but because of this bug some advancements got ruined.
The game changes every word in the file that says "world" to the name of the realm.
In my case our world name was "The Cockpit", and every advancement that had the word "world" was changed to "The Cockpit (Word 1)".

HOW TO REPRODUCE:
In order to reproduce this bug, open a singleplayer world, add a datapack that contains the word "world" in it, (I suggest the blazeandcave's advancement pack because it has many files with this word), then, upload your singleplayer world to a realm, give yourself all the advancements and then check to see if you have the "Explorer of worlds" advancement located in the Super Challenges tab (just an example of an advancement with the word world)
Upload your realm world to a singleplayer world, and check to see if you still have those advancements.
Additionally, you can open up the world file after you have downloaded it from the realm and search for your realm name, you should see it in places where the word "world" should be.