Wolrds glitch if you load them through 2 versions or more

When you load a world in old mcpe and upgrade it, chunk glitches will occour, why does this happen? Also un loaded chunks happen (they exist but textures dissapear in these areas) affects 1.14 but is fixed in 1.15 but did not happen in 1.13 (gone from 0.9.0 to 0.14.3 to 1.14.30)