Why is my mac displaying a grey loading screen on wake up?
What you're seeing is to be expected if your Mac is waking from what's called Safe Sleep.

Safe Sleep is a protective measure used by your Mac to ensure any data stored in memory isn't lost when your Mac shuts down due to a flat battery.

My guess is that the reason this only started about four months ago is that either:


the overall condition of your battery has changed
the power adapter has developed a fault
the way you leave your Mac at night has changed


In other words, for some reason, starting four months ago your battery is being drained to a level that data stored in your memory will be lost because your Mac is going to shutdown. So your Mac is going into Safe Mode to save all this data to your drive.

When it wakes up, you'll see the grey screen and progress bar that appears after Safe Sleep.

You can read more info from this Apple support article.

Now, in your question you state:


  Battery fully charged and still in good shape. Laptop is pretty much always plugged in. Full charge lasts all day, so I don't think it's a power issue. 


If you're absolutely certain of this, and have no reason to suspect a faulty power adapter, then I would reset your System Management Controller.

Reset the SMC


Shut down your MacBook Pro
Unplug the power cable from your Mac
For 10 seconds, press and hold at the same time the shiftcontroloption keys (on the left side of the built-in keyboard) and the power button
After 10 seconds let go of all keys and the power button 
Plug in the power cable
Turn your Mac back on with the power button.