I accidentally deleted a block I only wanted to delete on 1 side, on the other side this block didnt get placed (good) because a small generator was there. The small generator got deleted, but undo option in N menu did not work on it. This also resulted in an invisible block/ unuseable space on that spot. it is the 1 block above the 'blue' one in the screenshot. On the other side of this spot was the block I deleted.
I will try to rephrase what I said above.. place different sized devices or blocks on mirrors sides. on 1 side I had the small genny, other side a normal block in this case. I deleted the normal block (to put in a ventilator I think) On the other side the generator got deleted. Undo did not work to get my genny back. I could get the block back.
Don't know if this helps, but I'm having the same issue. I've dead space from a mirror deleted O2 storage. You can place a 2x2 device (e.g. O2, Fuel, Small genny etc) here: But not here: And a larger device won't fit either: Odd thing is, you can place a smaller block there just fine so it's actually just one dead block: I can remove a block behind and the 2x2 device fits, so essentially, this is the dead block bordered in pink: Cutting the space out with the debug menu has no effect - so not an invisible block. Saving, exiting and reloading has no effect. BUT WAIT! If you save a blueprint and re-spawn, it fixes the problem: If it's of any use, I can provide a game save with the dead block.
I deleted the fridge on the left on accident, lost the spawn point on the right.. resulting in a invissible/occupied block, the one bottom right where there is no plastic block. Undo didnt work to get the spawn point back. when I deleted the fridge again with mirror mode on, the invissible block was gone too.