One hardcoded (mdl_) prefix was introduced by
MDL-38378 and CI servers weren't able to detect that, mainly because mdl_ is the prefix they are using, so table was found, and upgrade ran without problems.
This is about to modify all the jobs using DB in the servers to start using another, non standard, prefix. So any "mdl_" prefix will lead to error here or there in the jobs (installation, upgrade, unit tests...)
That is, ciao