dead plants, falling sand/gravel/etc

Dead Plants

If there are lots of flowers, carpets and such, that are on your build, unless you have set the tickspeed to 0 using /gamerule randomTickSpeed 0 then they could get destroyed when you join the world.

Copy paste not working because plants and stuff keep dying/dropping (copypaste) If you are trying to copy and paste something and the flowers (for example) keep dying or dropping, you can fix this by simply: //perf and turn everything off. Then /paper fixlight to fix the lightning.

Falling sand/gravel/etc

If sand is falling after loading the build, try installing the FAWE (https://intellectualsites.github.io/download/fawe.html) plugin in the first place. It should prevent gravel, sand, etc from falling.