While riding a donkey or mule with a chest if you put items in the donkeys inventory and it's full your items just disappear

While riding a donkey with a chest attached if you open inventory and move items into the donkeys inventory and it's full your items will just disappear. I right click and shift to put items in and his chest was full so they just disappeared. This has happened multiple time. On Windows 10 PC.
Thanks,
Shannon