I tried replacing the realm with a world on my Nintendo Switch and it shows an error. The whole world in its entirety is not copied. However, upon downloading the world, it shows that the seed of the world I used to replace is identical to the downloaded world's.

This is only true on the Nintendo Switch. Meanwhile, I tried it on my Windows 10 version (which was already on 1.16.1) and the replace world function was flawless.

I tried it again on my Android version (which is also in 1.16.1) and though it shows an error like the Nintendo Switch version does, the world is still replaced, intact. 

Also, both Nintendo Switch and Android says that the level export was finished, but shows an error too.