In version 12, I traced it back to v12.0.17, there was changes to frappe/utils/pdf.py to make provision for wkhtmltopdf version update 0.12.5, I fixed the issue by updating wkhtmltopdf to 0.12.5.
On Ubuntu 16.04 server
$ wget https://github.com/wkhtmltopdf/wkhtmltopdf/releases/download/0.12.5/wkhtmltox_0.12.5-1.trusty_amd64.deb
$ sudo dpkg -i wkhtmltox_0.12.5-1.trusty_amd64.deb`
$ sudo apt -f install
On Ubuntu 18.04 server
$ sudo apt-get install -y software-properties-common && \
sudo apt-add-repository -y "deb http://security.ubuntu.com/ubuntu xenial-security main" && \
sudo apt-get -yq update && \
sudo apt-get install -y libxrender1 libfontconfig1 libx11-dev libjpeg62 libxtst6 \
fontconfig xfonts-75dpi xfonts-base libpng12-0 && \
wget "https://downloads.wkhtmltopdf.org/0.12/0.12.5/wkhtmltox_0.12.5-1.xenial_amd64.deb" && \
sudo dpkg -i wkhtmltox_0.12.5-1.xenial_amd64.deb && \
sudo apt-get -f install
This resolved the issue for me I personally tested both, I hope this helps