For ex. 1.8 world to higher version (1.18+)
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.
Good to know: When blocks fall, like sand or gravel, it uses what is called a "block update" which is where the server checks blocks for activity and if anything needs to be 'updated' around it. This takes precious CPU time to recalculate each block. FAWE bypasses block updates which is why it is so fast. Therefore, sand and gravel placed by FAWE will not fall and this is by design and it will not be "fixed". - mikedmd