Was able to confirm. Believe I see what's causing it. Will have it fixed for a new build today.
forge.joomlapolis.com/issues/6239
Seams to happen when a frontend category store is triggered during a group store, which appears to be from GJ Forums when it's generating the Kunena forum categories. So should be hard to trigger this regularly. You can workaround this by re-saving your categories so it generates their forum categories before a group save does it.