This is what it should look like ( from 1.10 multiconnect )

This is what it looks like in 1.9 multiconnect

And even though it's in the 1.8 branch, this is what it looks like with 1.8 multiconnect

This is what it looks like in vanilla 1.9/1.8

Tested on minehut. I imagined it'd be a problem with viaversion but i couldn't reproduce with paper using viaversion.

What block is that? That looks like air in vanilla, not flowing water. It could be block 36 or barrier.

Seems to be the structure void block

What is it in vanilla 1.9 and below? The structure void block was introduced in 1.10

It doesn't show as anything just the block behind it, although I don't think the server is 1.8.9 because if I do /version it tells me its using version 1.16.4 so it might be using ViaBackwards

If you do /plugins it shows you that it uses the 3 via plugins ( i believe forks )

If I were to guess, if a 1.9/1.8 client is presented with chunk data containing blocks it doesn't understand, it replaces them with air. So ViaBackwards relies on this behaviour and leaves structure voids untranslated. However when multiconnect receives blocks that the server shouldn't recognize, that is treated like undefined behaviour, hence what you're seeing.

The question now is whether multiconnect is wrong to treat this as undefined behaviour, or whether ViaBackwards is wrong not to sanitize this data.

Just looked at the ViaBackwards code, it seems to map structure voids to block ID 287 for some reason, but that block ID doesn't exist, so my hypothesis was almost right.