I had to deploy an EL8 based machine to get to the bottom of this. Turns out we missed something in packaging and on EL8 some files got deployed into the old scl prefix and therefore weren’t picked up by the running processes.
As a workaround deploy a symlink putting the file into the right place with
ln -s /opt/theforeman/tfm/root/usr/share/smart_proxy_dynflow_core/bundler.d/foreman_remote_execution_core.rb /usr/share/smart_proxy_dynflow_core/bundler.d/
and restart smart_proxy_dynflow_core