Right prompt not displayed

[romkatv]

To reproduce:

docker run -e LANG=C.UTF-8 -e LC_ALL=C.UTF-8 -e TERM=$TERM -it --rm ubuntu bash -uexc '
  cd
  apt update && apt install -y zsh git
  git clone https://github.com/bhilburn/powerlevel9k.git
  echo "
    POWERLEVEL9K_PROMPT_ON_NEWLINE=true
    POWERLEVEL9K_LEFT_PROMPT_ELEMENTS=(dir)
    POWERLEVEL9K_RIGHT_PROMPT_ELEMENTS=(custom_text)
    POWERLEVEL9K_CUSTOM_TEXT=\"echo \${(pl.\$((COLUMNS/2))..-.)}\"
    POWERLEVEL9K_MULTILINE_LAST_PROMPT_PREFIX=\"\${(pl.\$((COLUMNS/2))..-.)}> \"
    source ~/powerlevel9k/powerlevel9k.zsh-theme" >~/.zshrc
  exec zsh'

Expected:

Actual: