Tests are only supposed to run for changed plugins, to cut down on CI time and cost, and reduce issues from flake. That's not happening for the new macOS LUCI tests; they are running everything even for PRs that affect only one package. In flutter/plugins#6519 for instance, for commit 7107c17 in the PR, the macOS LUCI test run starts with:

Running for all packages that have diffs relative to "7107c17e4d91be6487e5a6b2242e42d34df032f9"

No changed packages.

It's using the last commit instead of computing the correct base to diff against. I'm not sure yet if this is something we need to fix in the recipe and/or task scripts, or whether the logic in flutter_plugin_tool is too fragile and was relying on something Cirrus-specific, so I'll need to investigate further.