We have same issue using Git with Multi-SCMs. Changelog stops listing detailed commit information for any jobs using the Multi-SCM plugin. I think this is a bug in the way multi-scm saves or modifies the changelog.xml, or in the way Jenkins Core parses and displays the information in the web GUI.
I don't think this a problem with subversion or git plugins. Just between jenkins core and the multi-scm plugin and handling of changelog.xml file.
Edit: another thing I just noticed in the screenshot, the SHA-1 hash for the 'unknown' commit has the word 'tree' on the end which shouldn't be there, so potentially an issue related to line endings or encoding in the changelog.xml?