I have Join and Copy Artifact plugins installed.
Test job has:

1. no Join Trigger configuration (Post-build Actions -> Join Trigger unchecked),
2. Copy Archiver configuration (Post-build Actions -> Aggregate the archived artifacts) with some "Shared directory" set.

Bug: when saving test job configuration "Shared directory" became empty.

As I see, this is because Join plugin inserts into page HTML code <input> tag with same name as used in Copy Archiver configuration HTML code (same <input> tags used, I think, because Join trigger inserts Copy Archiver configuration HTML code directly in his configuration).

Workaround for this bug is to manually update resetted <sharedDirectoryPath> item in job's config.xml

This is duplicate of JENKINS-5921 but for Join plugin.