To prevent flowers, carpets, and other decorations from being destroyed when you join your world, ensure the tickspeed is set to 0 using the command "/gamerule randomTickSpeed 0". This effectively pauses block updates, including the decay of plants.
On a Server
If you're on a server, you can execute the "/gamerule" command directly in the console to adjust the tickspeed.
Single-Player World
In a single-player world, you can set the tickspeed before loading the world or utilize mods to achieve the desired effect.
Copy-Paste Issues
If flowers or other items keep disappearing while copying and pasting, temporarily disable performance optimization using the command "//perf" and then fix lighting issues with "/paper fixlight". This should resolve the copy-paste problem.