Before we jump in on that.
When you push the emergency brake, does it push in tight nearer the top, or is it a longer push nearer the bottom?
Nearer to the bottom "might" reflect the self adjuster(s) are not adjusting the shoes outward as they wear.
Normally crud might keep them from moving.
Could be this part of the hardware was not replaced or cleaned well enough for reuse.
Note: One side has a right hand thread, the other side is a left hand thread.
Even stupider, if the right side adjuster screw got swapped for the left side. This would not let the shoes adjust outward at all when they need to.
"Wearing In" would mean the shoes mate to the drum snugly, like 100% of the lining surface is touching the drum.
If adjusting (or lack of) is not taking up the slack or gap as the shoe wears, lining contact to the drum will suffer.
Lesser friction (lesser stopping power) and since the "gong effect" of the drum is not dampened by full shoe contact, this could mean a groaning sound.
When I first read your question, I took it that you had replaced the drums after you heard the sound for a while. After reading it again, looks like this has been going on after the shoes should have worn to the drums curvature.
If you cannot access the innards (maybe send pictures).
If you call the shop and express/explain your concerns, they might crack them open and re-check their work?
Same chain replaced my elderly neighbors turn signal at no charge at all.
I am sure the dealership would not have been so generous!
I did post a few diagrams/screenshots that you can look over. The are Taurus. The last in the bunch has the hub removed (can see the adjuster better).
Images (Click to enlarge)
Monday, March 5th, 2018 AT 8:34 PM