Upon updating a world containing items with command block data loaded from 21w03a to 21w05a, the world works as normal until an attempt to save is made. At which point, the game will not successfully save. On terminating the program and attempting loading of the same world, the world will generate blocks and chunks up to a certain point around where the player is standing, upon which it stops generating them. The player can interact with their inventory, but cannot input any chat commands or interact with any blocks.

I have recreated this twice, using a command block to generate an item with some lore text, and then saving and reloading. No new (in this snapshot) blocks were placed or loaded.