The bug

When upgrading an old world containing connected redstone dust to 21w43a, often, the block states of the redstone will be mixed up. This causes crosses to often appear in a chain of connected redstone, where one may expect the redstone to only have connections to neighboring redstone.

How to reproduce

1. Create a world in an old version (I used 1.11.2)
2. Place redstone around, ensuring there are some connections in which only some sides are connected to other redstone (for example, a straight line)
3. Upgrade the world to 21w43a
   Some/all redstone has converted into crosses where it should not have

This may take a few tries to reproduce.

Observed result

Some/all redstone has converted into crosses where it should not have.

Expected result

The Redstone wouldn't be the "+" shape when converted when it isn't necessary.