Comment 13 for bug 1734132

Sorry, I should have tested that first before sharing the instructions…
There you go, with those you should be able to downgrade:

    sudo apt remove "libreoffice*"
    sudo apt autoremove
    sudo apt install \
        libreoffice=1:5.4.1-0ubuntu1 \
        libreoffice-avmedia-backend-gstreamer=1:5.4.1-0ubuntu1 \
        libreoffice-base=1:5.4.1-0ubuntu1 \
        libreoffice-base-core=1:5.4.1-0ubuntu1 \
        libreoffice-base-drivers=1:5.4.1-0ubuntu1 \
        libreoffice-core=1:5.4.1-0ubuntu1 \
        libreoffice-calc=1:5.4.1-0ubuntu1 \
        libreoffice-common=1:5.4.1-0ubuntu1 \
        libreoffice-draw=1:5.4.1-0ubuntu1 \
        libreoffice-gtk3=1:5.4.1-0ubuntu1
        libreoffice-impress=1:5.4.1-0ubuntu1 \
        libreoffice-math=1:5.4.1-0ubuntu1 \
        libreoffice-report-builder-bin=1:5.4.1-0ubuntu1 \
        libreoffice-style-elementary=1:5.4.1-0ubuntu1 \
        libreoffice-style-galaxy=1:5.4.1-0ubuntu1 \
        libreoffice-style-tango=1:5.4.1-0ubuntu1 \
        libreoffice-writer=1:5.4.1-0ubuntu1 \
        python3-uno=1:5.4.1-0ubuntu1