someone sliced it out? i would get matching bricks place those so they bridge gap then apply proper compound to cement in place.

i don't know, i have never seen a retaining wall that did not start to give after a while. perhaps on commercial projects but on home ones..... anyway have had to do a couple over the years, that is what we do. repoint the brick, remove some of the excess matter, which they all get. like i said yours looks like someone cut it though.

Hydrostatic pressure is causing it to move. It is not properly built and is a risk to the integrity of your foundation as is. It needs to be rebuilt properly. That means an engineered solution, not a mow your grass landscape type guy.