Hi experts, has anyone ever encountered this problem:

SLX brakes, aftermarket disco ceramic brake pads, dry conditions.
Front brake works great but 5 months after installation the rear break developed this problem:

When starting a ride the rear brake has little power, howls like a banshee, and has what I imagine to be a glazed-over feeling, but after 20 or 30 hard applications it starts behaving and grabbing very well. The next ride the process repeats itself. Same pads on the front with no issue.

Pad contamination is not making much sense unless heating the rotor through application changes the contamination phenomena. I pulled the pad and inspected it and couldn't see anything out of the ordinary except for slight pitting in small bands near the top and bottom of the pad. The pad was about half consumed.

I replaced the pads so we'll see but I was curious if anyone has run into this before. Also, how does one know that the pad needs to be replaced before getting down to the base plate which I assume will harm the rotor. thanks.