For awareness, I wanted to point out that tripleo-ci-centos-8-containers-multinode is now using ephemeral Heat on the master and wallaby branches. This change[1] merged yesterday that made the switch. Of note, the Heat logs for the overcloud deployment are now under the directory undercloud/home/zuul/overcloud-deploy/overcloud/heat-launcher/log in the job logs.
There is also a pending docs patch that covers a bit more how the feature works: