Same issues here from new. Mine are Ford factory manual reclining seats. Haven't addressed it yet but did let Ford look at it once at the dealership. He folded the seat all the way forward, reached underneath seat from the back and the "cover" for the back of the seat is velcro'd. He loosened it and folded it upward, allowing his hands to run underneath, up to the area where the armrest goes in. I'm guessing there's some type of retainer ring under there. Your post has encouraged me to go back now and finish this issue. It hasn't been too much of problem for me as my Lab likes to sit between the front seats...first thing she does is put her nose under the armrest and fold it up so she can use my lap for a pillow. Kinda funny...a few times when she was in the back, I would fold the armrest down...she would come up there, push it back up and then head to the back again. I quickly realized it wasn't my MH...I bought it for her.
2018 THOR Chateau 35SF
Two Labs, Bugsie & Blondie