The code in the TestExpectations style checker only lints the test expectations file against the port it is running on. It really should be linting against all ports, like new-run-webkit-tests --lint-test-files does. The easiest fix is to just have it call into the lint routines in webkitpy/layout_tests ... I should probably refactor that logic out of TestRunner and into test_expectations.
There's no point in fixing this before we fix/land 51222, since all of the logic changes in that fix.
Bug 51500 gave a false positive on the style bot because of this (as an example).
There's also a lot of testing duplicated between the style checker and the unit tests for the expectations parser that's kind of unnecessary and can lead to confusion (see, e.g. bug 48426 for an example where one set of tests was updated but the other wasn't).
I'm pretty sure I fixed this a while ago. It seems to e be working now.