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 < 21.08 displays the whole double height characters.
What did real VTxxx do?