Since v3.0, using variables in the "Rollout %" field of a Freestyle job has been broken.

If you enter a variable as the rollout percentage, e.g. ${ROLLOUT_PCT}, Jenkins would attempt to coerce this to a Double value when saving the job, which apparently does not crash, but instead returns null.

Since the plugin interprets null to mean "the default value", entering a variable into the rollout percentage field, saving the job, and then opening the job config again would show "100.0" as the value.

i.e. if you attempt to use a variable, your builds will end up quietly ignoring the variable, and will roll out to 100%.