The escape sequences
ESC # 3 and
ESC # 4 (DECDHL) are meant to be used on pairs on adjacent lines to request double height lines. As an example:
echo -e "\e#3DOUBLE HEIGHT LINE\n\e#4DOUBLE HEIGHT LINE"
Now different terminal emulators behave differently when only a single line is set to double height. xterm happily displays the corresponding half of the double height characters, while konsole displays the whole double height characters.
What did real VTxxx do?