I think this started in 1.16.100. - But with all the other bugs it introduced in 1.16.100 it was a little under the radar. On the other hand it may have started to occure because I built up the chunks, placing lots of glass blocks which will stress the system. I only play realms so this may behave differently with the game hosted on a personal device.

One half of a Large Boxe does not always load if one half of the box is placed in one chunk, and the other in a second. It feels like about 1 in ten chance, but not keeping a real good record. Accessing the box will often cause a crash of the game on my IPAD, but not a crash of the realm. Work around is of course to not have boxes located in two chunks.

This can often be fixed by running far enough and coming back that the chunks to reload. Or doing anything else that causes the chunks to reload. However it can happen several times in a row.

To replicate probably the quickest, after placing the large box in two chunks build a portal and go in and out of the nether. 

To visually see the bug easier place a item frame on each end of the box. If the half of the box it is attached to does not load you will have a item frame floating in the air not visually attached to anything. When it occurs this will not occur with just one box, but all boxes straddling the two chunks. Maybe having more than one box in two chuncks will cause it to happen quicker - definitely be easier to see.