I'm a bit frustrated by this bug. I have had this happen quite a few times now, but never filed a report because I can't make it reproducible.

I'm on Alpha 17.4, Windows.
With or without EAC seems to make no difference.
Restarting the game client makes no difference.

Sadly as I said I can't give you an exact way to reproduce this, I'll simply describe what I did today.
All previous encounters with this bug were similar: Large structure collapses, SI bugged when trying to rebuild.

1.:
Built a large structure (concrete), a bridge with support pillars 15 blocks high, support pillars come in pairs of two that are 3 blocks apart and the pairs are 10 blocks apart from each other, the bridge itself is a single layer of concrete on top of that. Structurally that is no problem whatsoever on its own.

2.:
I wanted to test how much of the structure would collapse if zombies managed to take out some of the supports.
Tested that multiple times in different variations.
That much collapsing seems to have corrupted the game state, some blocks are simply broken (map location, block type does not matter it seems).
So for example I have this one block at elevation 15 or 16, where one of the support pillars was.
When I rebuild the pillar, I can place that block, but it apparently provides no vertical support (even tho it has a direct line to bedrock which should give it perfect integrity I would think).

https://imgur.com/a/JQvdZXl <- the pillar in question, there are no caves below it - I checked, though I believe that it wouldn't have worked in the first place if that was the issue
https://imgur.com/a/5KKhaPC <- the block that I'm trying to place here on top of the structure is not placeable

Now here's the funny part:
If I put something with vertical support NEXT to the block that currently does not support any weight, it becomes fully functional again:
https://imgur.com/a/yHhhokf

Hope that helps and maybe you can track that problem down eventually. As I said, that has happened a few times to me now and always on my big building projects. Which makes it extremely frustrating. (You can't easily move a 5000 block structure by 20 blocks simply because one block decides to be buggy ....)