It sounds releng's TrainBranchBot has started to merge code to mediawiki-config master branch, causing undeployed code problem.
@Catrope described why this is an issue in https://wikitech.wikimedia.org/wiki/How_to_deploy_code#Problem:_undeployed_code. Long story short, it confuses any deployer who deploys after.
With humans causing this issue, it is usually resolved with reverting the patch and (sometimes) by yelling at people. With bots, this is much more serious -- as the bot will just do a new patch after it is reverted.
For that reason, I've removed the ability of TrainBranchBot to merge there by removing it from wmf-deployment Gerrit group, until this issue is resolved.
Since this probably breaks other parts of the bot (namely, branching), I fill this as a new UBN task.