Try to put a shulker box in the saved hotbar, in the previous snapshot with for example a painting of variant "earth" in it, along with a bunch of 'normal' items.
Now load the snapshot. The whole shulker box is empty. with the log error:
[21:36:03] [Server thread/ERROR]: Tried to load invalid item: 'Missing id for entity in: {variant:"minecraft:earth"} missed input: {"minecraft:entity_data":{variant:"minecraft:earth"}} missed input: {"minecraft:container":[{item:{components:{"minecraft:entity_data":{variant:"minecraft:earth"}},count:1,id:"minecraft:painting"},slot:0},{item:{count:1,id:"minecraft:wither_spawn_egg"},slot:14},{item:{count:1,id:"minecraft:ender_dragon_spawn_egg"},slot:21}]}'
I would at the very least hope that it only fails to load the one item, (even though in this case it shouldn't even error at all, or give me the default painting if anything)
This will also occur with bundles as well as shulker boxes.
Notably, this can also be triggered when overstacked items are present within each.
I've also attached an example world so the effects can be seen when updating.