i am on bedrock.
There are a total of 9 overworlds.
If you have not unlock 9 of them, and you are expanding.
Here are some of the situations that you will bump in:
1) blocks can be place (item deduced from your inventory) and the blocks will not appear. It is like placing an invisible block
2) you are unable to place the block
3) you are able to walk through as tho as there is a block (but there is no block). You either fall off into the void, or stuck there and get health deduced.
If you have unlocked the overworlds, at the border of the world, you cannot build and cannot walk into void. Items / mobs are still able to drop into the void.
I hope you can understand my poor english.