With the latest RecalBox image, I was having issues with a brand new install on RPi3b ignoring overscan settings (always bleeding off top and bottom) on a 720p monitor in both interface and MAME.
Commenting out everything in boot/config.txt and adding this one line fixed it (there didn't seem to be any difference with any of the other settings I tried):
I also suggest adding this to https://github.com/recalbox/recalbox-os/wiki/Overscan-settings-(EN)
**Note that if bleeding off of top / bottom of screen only, a single setting may be used. Comment all out all other settings pertaining to overscan, and add this one setting: