Parameters which are set in properties in a pipeline build are null the first time the build is run, even if they are given default values. On subsequent builds, the parameters have values as expected.

Example: create a new Pipeline job and define the pipeline script like this in the build config:

properties([
  [$class: 'ParametersDefinitionProperty',
    parameterDefinitions: [
      [$class: 'StringParameterDefinition',
        name: 'SOME_PARAMETER',
        defaultValue: 'some value',
        description: 'A test parameter']]
  ]
])

echo "Value of parameter:"
echo env.SOME_PARAMETER

Run the build by clicking "Build" (note that this says "Build" rather than "Build with Parameters" unless the build has been run at least once). The console output is:

[Pipeline] echo
Value of parameter:
[Pipeline] echo
null

Run the build a second time by clicking "Build with Parameters" and then confirming the default value. The console output is now:

[Pipeline] echo
Value of parameter:
[Pipeline] echo
some value

If you refer to the parameter directly using SOME_PARAMETER rather than env.SOME_PARAMETER, the first build fails with a groovy.lang.MissingPropertyException instead.

This isn't so bad when you're configuring a single new job, but it becomes a real problem for multibranch pipeline jobs, because the first build of every branch always fails unless the pipeline script does extra work to handle the missing parameter.