I noticed this myself and asked Roku about it:
There are currently limitations on using screensavers in conjunction with roScreen apps. Please see the v3.0 docs for more details.
except I can't find anything about "screen savers" or "screensavers" when text searching all the Roku docs.
My assumption is they are referring to the first paragraph of the RoScreen section of the Brightscript manual, which still doesn't answer the question, or there is some unreleased doc that they are looking at that we don't have access to.
Next, I'm going to try seeing if I can write to a different Z plane and see if anything shows up.
I've also seen this issue on Roku2 with Netflix or the Channel Store open - seems to not occur (last time I checked, anyway) on the original series with 3.0. Also some weird but cool fx where the underlying grid screen peeks through occasionally...
EDIT: I see your post was about the opposite: an roScreen app instead of a screensaver. Well, anyway, still, the issue is more or less the same...