This is the same issue I reported for 1.20.71 and was supposed to be fixed in 1.21.  The original bug report is REALMS-11707. 

My son and I both have been unable to break a brick with the normal one-click method since version 1.20.71. If either of us click the break button once, the brick will disappear for a fraction of a second and then re-appear. If we want to destroy a brick, we must hold the button down for roughly 1 second before the brink will be destroyed. This bug is on my son's account and my account when playing his shared realm.

However, if he leaves his shared realm and goes to another world purchased on the marketplace, the single button push will destroy blocks. If I go to my own world, I can also destroy bricks with a single button click. I do not have to hold the button down for 1 second before the brick is destroyed.