Update:
I deactivated all plugins, with exception to AO and left the Optimize CSS Code box checked. I also used a different WP theme and paused Cloudflare. Ran the Google Mobile-Friendly Test and it passed.
After activating one plugin at a time, my Metal Slider plugin seem to be in conflict with AO. The Google Mobile-Friendly Test failed. However, after all plugins were activated, I Emptied All Cache and Updated Media Query String in W3TC, as well as Save Changes and Empty Cache in AO. I also checked the other two boxes Optimize HTML Code and Optimize JavaScript Code in AO - the way I always had it.
The Google Mobile-Friendly test passed which didn't pass after many attempts, before posting here. I thought initially the Metal Slider plugin was causing the issue but after running the Google Mobile-Friendly test several times and each time it passed, I'm somewhat at a lost.
Next, I resumed Cloudflare which is now active again for my site and ran the mobile test several times. Each time the test pass.
Note: I have clearing the cache in my Mozilla browser all along when using the Google Mobile-Friendly tool.
I'm going to assume this issue will arise again where the Google Mobile-Friendly test will fail. I suppose following through what I did today will correct the issue if it should occur again. I have the distinct feeling something is going on with Metal Slider and AO optimize CSS.
For the moment, this issue seems to be resolved.