I do have plugins, but they’re pretty common ones:
The only unknown quantity here is “Watch Category” but it doesn’t do anything relevant. To make sure, I confirmed that the issue also appears on my local Vagrant server (also on v1.6.0.beta10). I removed all plugins and confirmed that issue remains. (I didn’t want to try this in my production server )
Another interesting point. From a sub-category I can select the parent category and other sub-categories from the same parent:
Sorry for the thin initial report. I’m a bit buried with work and I posted too eagerly.