After I installed a fresh instance of phpBB 3.0.8, I registered another new user ("bar") and used the default founder user ("foo") to set "bar" to also be a founder. I also added "bar" to the Administrators group.

While logged in as "bar", I was able to create new forums, but I discovered that I could not properly move them (i.e. set their parents) to other forums I had created. It was then that I discovered that my user-specific permissions (including the admin permissions) were all set to "no". (Apparently I was allowed into the ACP and all that based on my membership in the Administrators group?)

Setting all of bar's personal permissions to YES allowed me to move forums as expected.

It seems checking the Founder box in the user ACP does not carry over all the proper founder permissions to the newly-crowned founder.