Today I tried to push this change: https://gerrit.wikimedia.org/r/#/c/338326/ and it quickly got -1 from Jenkins bot.
After a few tests trying to rebase the change manually, and remove the tabs and so forth, there was no way it would get +1.
@Volans and @jcrespo also tried to debug the issue without finding anything.
In order to discard issues with that specific commit and/or file we tried different test commits even on other files, and they always got -1:
We forced a +2 and it got merged successfully so looks like cobalt isn't the issue but the copy the jenkins bot is using.
After getting it merged, a git pull on another old copy shows the merge and the file without any issue.
The reason for setting this priority as High is because it might be really confusing as you wouldn't know if your commit is wrong and can get confused looking for issues on your code while the problem might not be it.