Items in hand disappears when other players join the realm

This has been happening frequently, some of my items in hand started disappear all of a sudden. And then I figured out that this happen when a player join in the realm's world. I tried to do a test, I selected some blocks in hand and asked to my cousin (one of the members of my realm) to join the world, and when her minecraft started loading to join (that purple portal nether screen) my item disappeared from my inventory! I tried it again, but this time my cousin was the one with the blocks in hand to run the test. I joined the realm, and her block disappeared too. It happens ALWAYS to our realm when someone joins. I haven't found any issues about this here, so I decided to ask for help.