See #3164686: WebAssert::addressEquals() and AssertLegacyTrait::assertUrl() fail to check the querystring → for the core change and https://www.drupal.org/pift-ci-job/1804842 → for the Flag test results.
I also recommend turning on D9.1 testing for Flag (maybe weekly testing), so core changes like this can be caught and fixed when they happen.
Closed: outdated
4.0
Flag core
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