Space limit / meteor debris broken

I walled off my space border like this:

Now I am set up to remove that barrier:

I am left whit this and dupes can mine two tiles into the space barrier to remove it:

But there is stuff left that cannot be selected for digging and hence cannot be removed by dupe:

Interestingly, the auto-miner can remove it:

There is clearly some problem with space debris piling up into regions the dupes cannot mine.

Attached save is the state from the fist picture.