I have noticed this when I was working on the issue:
It looks like that if you have multiple versions of the same plugin, only one of them can be installed as the plugins and models maps are used with the groupId.artifactId as the key.
So a subsequent version of the same plugin will replace the previous one leading to the last-one-wins situation.
This might look an innocent bug however image you have two set of plugins in your installation. Then you will end up a randomly -picked list of a single entry list of the plugins in your repository that can possibly lead to unwanted version chaos. Furthermore this situation can be missed easily and can cause a big headache to the experiencing person.
Attached path resolves the issue...