When you supply the size and stacked props to a b-form-radio-group, the group's element is given a fixed height (incorrectly) by the form-control-${size} class, meaning that the child b-form-radios are clipped, or they obscure any content below.

OS: Ubuntu 18.04.2 (although I'm sure it applies everywhere)
Browsers: tested on Firefox 65 and Chromium 71 (but, as above)
BootstrapVue version: 2.0.0-rc.13
Test case: https://jsfiddle.net/markrian/96Lbqwyx/10/

You can also see this happen if you go to the docs and add size="sm" or size="lg" to the stacked example.