Sometimes the artifact copy step is skipped for unknown reasons, resulting in a bad promotion.
I can locate no useful logs to identify the issue, other than the promotion log itself showing a missed critical step.
All promotions are manual, and run a shell script as the actual promotion.

Log head from an incorrect promotion example log:

Started by user ilijab
Building remotely on buildslave4 in workspace /var/lib/jenkins/workspace/esensor-core
Promoting esensor-core #93
[esensor-core] $ /bin/bash -xe /tmp/hudson5568737232874898866.sh
. . .

Log head from a correct promotion of the same job, run earlier in the day with no product or build changes:

Started by user wfang
Building remotely on buildslave1 in workspace /var/lib/jenkins/workspace/esensor-core
Promoting esensor-core #92
Copied 2 artifacts from "esensor-core" build number 92
build hudson.plugins.copyartifact.CopyArtifact@144ec690 SUCCESS

. . .

As can be seen, the promotion sometimes copies the correct artifacts from master, and sometimes incorrectly decides not to. Since the promotion and the related build processes will run on an arbitrary host in the cluster, old workspace contents are generally present and it is not clear why it decides to wrongly skip the copy step.