I have hopped into the game to try it myself. What is going on is the following:
you have created a ladder under the bedrock. To create a ladder normally, you need to press B on an empty ground layer. this will take you to the underground section. You can get back up to the surface by pressing B on the ladder. Since there is no ground underneath the bedrock and it doesn't perform any checks for whether or not there is a block in the way, it sends you down there regardless. Going back up destroys the bedrock as it forces the block above the ladder to be air.
destroying the bedrock and allowing you to get out of bounds is a glitch. But the section you have been sent to is intentional and required to progress.
This will, however, not be fixed. The code is a complete mess and would require fully rewriting everything. I do have plans to do that anyway so that will be a good opportunity to fix it.