MacBook Pro 15'' RAM Question
The 2011 Mac MacBook Pros have an Apple specified (as you found) maximum amount of RAM as 8GB.  The "real world" tests all show that the 2011 MacBook Pros actually have a maximum amount of 16GB of RAM.  See below for several examples.  This has been somewhat common practice for Apple over the years, where there specs will show a maximum of X but the real world tests and reviews all confirm a maximum of Y.

Your initial 8GB DIMM was probably faulty.  Had it been functioning properly your Mac should have recognized all 16GB of memory.  There are no technical reasons or firmware restrictions that would keep your Mac from recognizing the full 16GB.

Other World Computing's RAM upgrade page for this machine

Crucial's RAM upgrade page for this machine

EveryMac's spec page for this machine showing official &amp; unofficial RAM specs