@Windsdon, this report is only about block positions and entity UUIDs not being written and read correctly, the (outdated) list in the description only mentions cases where the UUID would have to be adjusted.
For passengers not being loaded, see MC-103452.
Windsdon
MC-9553
Block breaking particles render over smoke (and possibly other particles).
Please see 2014-01-24_11.03.24.png
Can confirm on 21w18a, demonstrated here with a spider jockey. The spider loses the Passengers tag, so the skeleton doesn't spawn.