My pipeline defines a parameter with sth like:

parameters {
 booleanParam(name: 'RUN_TESTS', defaultValue: false, description: 'Do you want to run the build with tests?')
 }

Printing the param value with

script {
 if (params.RUN_TESTS) {
 echo "true: $RUN_TESTS"
 }
 else echo "false: $RUN_TESTS"
 }

shows that the value is always TRUE and the tests are always executed.

 I think this is somewhat related to https://issues.jenkins-ci.org/browse/JENKINS-36543