Brett Hand AC parameters have not been tested in use with Jenkinsfile and we only have experience using them in 'freestyle' type builds. Nonetheless, in these cases returning a list of Strings is required for proper display in the build form UI. The parameters are passed to the build as text with the selected options as a comma separated String. You may need to tokenize this string to return a list. Note that 'splitting' a string with Groovy returns a String not a list
I hope this helps. And any experience with using AC parameters in jenkinsfile is always useful (for future reference). Thank you!