Glitches and unconstructible spots

TreeToTheVoid

While digging my way out of a cliff, I found a lot of glitches and some spots where, for no apparent reasons, I could not build anything. Please find attached an illustration and the save game.
In the save game, you will find a tunnel going down the island and an arch going up from where the tunnel emerges. This arch ought to be way more large (as shows its base), but you'll find that the unused spots are not constructible.

DrWhoCares

I would not know, but this may have something to do with how islands stay afloat. I believe there are blocks that cannot be destroyed (as they are essentially the "bedrock" layer). I remember seeing something in the preset files about whether or not you can dig the bedrock layer. This may possibly be related? QA would know better.

Shatojon

Forwarded to QA, thank you