Okay, so a couple of things I would like to explain:
1. Difference between TileEntity::getUpdateTag and TileEntity::getUpdatePacket.
They are build around the same purpose and cause a lot of confusion, but they are slightly different. getUpdateTag is appended to the ChunkData packet(the one player recieves from a server when loading a chunk/chunk data is changed). getUpdatePacket is sent in 2 circumstances: If a BlockState changes and the new one has a TileEntity(world::setBlockState is mostly the cause here) or if the amount of changes in the chunk is greater than 1 and less than 64(so a couple of blocks got thanged). Then the getUpdatePacket is sent for each changed blockstate with tile entity.
So you do need to override both in your tileentity. Override only getUpdatePacket - and the client will not recieve the data when the chunk is loaded or multiple blocks are changed at the same time. Override only getUpdateTag - and the clients will not see the changes upon blockstate changes.
2. Please use breakpoints to debug. They are so much nicer and handy, you have no idea. Writing stuff to output is messy, time-consuming and leads to situations when modmakers forget to remove their println debugging from the final product and the log gets overflown with useless debug information and slows the game down. Just launch any big modpack out there and watch the log. You will quickly see what I mean. A breakpoint is literally 2 clicks of mouse to set, gives you all the info you need and even more and it will never end up in the final product.
3. getUpdateTag should return all your NBT information you need to sync. A couple of responses above you posted:
That will not work. It will send an empty packet to the server upon chunk loading and your tileentity will not have any data synced. Even worse, it will completely mess everything as the tileentity will not get loaded at all! See how it is done in the base Tileentity class.
If you simply override both of those methods and their handlers (onDataPacket, handleUpdateTag) then your tile will get updated correctly on the client when the respective packet arrives. You still will need custom packets to sync real-time changes though.