Unfortunately T13615 is indeed known to be fragile. This is tracked as #17269, ss you discovered. This is especially scary given that the test tests some rather subtle RTS logic. It's been on my list of things to investigate but unfortunately I've not been able to reproduce the failure locally so progress has been slow.
Given that it seems to fail fairly reliably on CI in some cases we probably ought to mark it as fragile.
Note that it's not quite true that fragile test outcomes are no longer tracked. We maintain a database of test outcomes from CI runs which includes both passing and failing ribs of fragile tests. I've written about this infrastructure here .