Well this happend in both versions, standalone and RetroArch. When switching to fullscreen, MAME crops the top and the bottom, so that you only see half of your score at the top, and f.ex. only half of the Insert Coin at the bottom.
I tried several settings, but it doesn't change. Resolution is set to 1920x1080, and I'm using it on a TV (set to 1920x1080), and not a monitor. I tried IntegerScale and CropOverscan, changed resolutions, custom aspect ratios, but nothing works.
Any ideas? Thanks.