VTK does not display isosurface on PyQt5/Ubuntu 20.04 - qt

Question

I'm trying to run a PyQt VTK application (Pegamoid), on a supercomputer server, for which I have to jump through some hoops, and at the moment I've settled for building a Singularity container. However, I find some strange display problem.
When I build a container based on PyQt4 and Ubuntu 18.04, everything looks fine:
Bootstrap: docker
From: ubuntu:18.04
%environment
export LC_ALL=C
export PYTHONNOUSERSITE=True
%post
export LC_ALL=C
export DEBIAN_FRONTEND=noninteractive
export PYTHONNOUSERSITE=True
mkdir -p /tmp/apt
echo "Dir::Cache /tmp/apt;" > /etc/apt/apt.conf.d/singularity-cache.conf
apt-get update && \
apt-get -y install --no-install-recommends python3-pip python3-qtpy python3-pyqt4
python3 -m pip install --no-cache-dir -U Pegamoid
%runscript
pegamoid.py "$#"
But with the equivalent for PyQt5 and Ubuntu 20.04, the isosurface (brown/yellow) does not appear:
Bootstrap: docker
From: ubuntu:20.04
%environment
export LC_ALL=C
export PYTHONNOUSERSITE=True
%post
export LC_ALL=C
export DEBIAN_FRONTEND=noninteractive
export PYTHONNOUSERSITE=True
mkdir -p /tmp/apt
echo "Dir::Cache /tmp/apt;" > /etc/apt/apt.conf.d/singularity-cache.conf
apt-get update && \
apt-get -y install --no-install-recommends python3-pip python3-qtpy python3-pyqt5
python3 -m pip install --no-cache-dir -U Pegamoid
# Fix strange bug
apt-get -y install --no-install-recommends binutils && \
strip --remove-section=.note.ABI-tag /usr/lib/x86_64-linux-gnu/libQt5Core.so.5 && \
apt-get -y autoremove --purge binutils
%runscript
pegamoid.py "$#"
Any idea about causes or solutions?
ETA: Using PyQt5 in 18.04 works fine too. Using PyQt5 on 21.04 and 22.04 given the same problem as 20.04.