Note:  this problem is reproducible with RadTextBoxControl as well. The Enter and Leave events also don't fire when RadTextBoxControl is inside a RadPopupContainer

Resolution:

Case 1:
RadTextBoxElement is not a control and do not have validation events. Use RadTextBox with RadMenuHostItem instead:

RadTextBox tb = new RadTextBox();
RadMenuHostItem item = new RadMenuHostItem(tb);
tb.Validating += Tb_Validating;
item.MinSize = new Size(50, 20);
radMenu1.Items.Add(item);

Case 2:

RadPopupEditor has message filter which is responsible for closing the popup and it intercept the leave and enter messages. Use GotFocus and LostFocus instead.