1. install the Jenkins Parameterized Trigger plugin (2.35.2)
2. create a freestyle job
3. click Add build step
4. click Trigger/call builds on [sic] other projects
5. click Add Parameters
6. click Restrict matrix execution to a subset
7. click
8. read the bottom of the help:

However, Jenkins variable expansion leaves undefined variables as-is, so most of the time your Groovy string line expression syntax will survive the expansion, get passed to Groovy as-is, and work as expected. If you do need to escape '$', use '$'.

Expected results: the two things should presumably be different.

The relevant code is:

https://github.com/jenkinsci/parameterized-trigger-plugin/blob/55f8d5b10f00164a090972f8af89794acddf870b/src/main/resources/hudson/plugins/parameterizedtrigger/matrix/MatrixSubsetBuildParameters/help-filter.html#L20

If you do need to escape '$', use '$$'. 

I'm not going to fix this right away. Filing it because I found it while reading the help for everything and I'm punting as I'm not quite sure about the fix.