As of the earliest build for 1.21 changed to advanced placement seem to result in hard client crashes without any crash logs. This only seems to affect quite a handful of mods, especially some of the top popular mods that utilize this system. The crash seems to appear when ever advanced placement is toggled and the hologram is created. Upon attempting to debug this issue via breakpoints and step through, I can only get as far as the player's command handler, before the game forcefully crashes without any error.
Here are a couple of the mods that are affected:
I have several confirmations from several modders that utilize the same general approach to advanced placement kits, where the placed item is not the same type as the held item. Please look into this before 1.21 launches to stable, as a workaround would be much needed if the core issue does not fit within the development cycle, as we're not able to properly debug this issue with the tools provided.