Realms handles some commands incorrectly. The example we found which breaks both Run Swim Fly Parkour as well as Moss Block Challenge is running two execute positioned arguments next to one another causes an error on realms when it shouldn't.

Partner Rank: 1
 
Verification builds:
1.19.81 Live Build
1.19.81 Realm Build 04/05/2023
 
Impact:
Players are unable to play TWO of our maps on Realms! This is an issue which will affect multiple maps from multiple teams, and will make players think the map itself is broken, when it's actually Realms which is causing the issue.
 
Repro Steps:
To see the bug:
Get Run Swim Fly Parkour from the Marketplace Store [https://www.minecraft.net/en-us/marketplace/pdp?id=d2b7718d-98e1-485a-a61f-a8a0885cdf26].
Create the world on Realms.
Press "Start Map".
Run down the stairs and to the right, then through the start gate.
Note that there is no timer over the hotbar, and the item which appears in your inventory does nothing when interacted with.
 
Here's a simplified way to check if this type of command works:
Create a local world with cheats and run the following command: /execute positioned ~ ~ ~ positioned ~ ~ ~ run say Working!
Open a world hosted on a realm with cheats enabled and run the same command.
 
If the command prints "Working!" in chat, that means it's working! If it doesn't, wellllll, then it isn't working...
 
Observed Results:
The map is not functioning correctly on realms compared to the local world. Several elements of the world are affected, including the Timer, Inventory Items, and Jump Pads.
 
Expected Results:
Realm command behavior should be the same as local world command behavior.
 
Screenshots/Videos attached: Yes
 
Regression Build:
Unknow, we can't test regression on Realms.