I found a solution here in the later comments:

https://retropie.org.uk/forum/topic/7843/boxart-screenshot-display-same-time/3

I had to add <marquee>./marquees/, <image>./images/ AND <video>./videos/ on each gamelist (duplicated multiples via regex) even though there's no video folder.

The presence of the video field enables for a marquee to be loaded instead, when <showSnapshotNoVideo> is set to true.