Well, in the case of the logos, I was adding those to a panel , which was then added to the CLV, and that panel was created programmatically; I didn't explicitly set its background color, as the page background was set to white anyway. I've just tweaked the code to explicitly do so.
However, before installing a new test version, I also updated the phone that was exhibiting this behaviour to 13.2.2 - and the problem's gone away with that update.
Still, I shall make sure I do check all the background colors carefully, just in case this happens again.