Thanks for the response Daniel.
I now see that this is not a folders plugin related issue, but appears to be a Jenkins version issue.
My initial efforts were on a Jenkins OSS version 1.569 using Rebuilders plugin version 1.21. This was a folder-less setup.
My folderized tests system is a Jenkins Enterprise version 1.574 using Rebuilders 1.21. This is the folderized setup.
Since the Jenkins versions were pretty close, i didn't expect different behavior based on the Jenkins version, but in fact that seems to be the case.
To simplify the test i made two new free-style jobs at the top level of Jenkins instances.
"Trigger Source" has one String parameter "TEST_PARAM" default value = "Got it?".
"Trigger Target" job is just a a new free style with no changes.
"Trigger Source" triggers "Trigger Target" with Add Paramters/Current build parameters.
After one time launch of "Trigger Source" on the 1.569 instance, doing Rebuild Last on the "Trigger Target"displays the TEST_PARAM passed in, but on the 1.574 Enterprise instance, rebuilding the "Trigger Target" job does not display or pass the previously passed in TEST_PARAM.
I'll do some additional testing on the Jenkins versions in-between these two, and also OSS vs. Enterprise to see if i can isolate where the change occurred.