So once again - in the same cycle of things - custom format strings that have %% (in other words desire to display format as a percentage without going the auto route) are broken and back to displaying %% without being correctly escaped.
This has been broken/fixed at least 3 times I can find by @raster @zmike @netstar and myself:
https://phab.enlightenment.org/rEFLb4112b9735ce75bce95974a257aa1880fa52a6bf
https://phab.enlightenment.org/rEFLa96933f964b1757b9d9f8a4cce14d883f35a6665
https://phab.enlightenment.org/rEFLe96532f65c13ffe11b25f7d9b384daa197d928e8
While the elementary_test has a progressbar test that displays this exact very behavior for the purpose of not breaking it - apparently only ninja tests are used these days so - once fixed - its apparent that a test should be made for it. Al has a task created for this it seems: https://phab.enlightenment.org/T7317