What's the little caption in the corner?
When a new piece starts, your frame shows its title, artist and year in the lower-right corner for about 10 seconds. Your own photos only get a caption if you've given them a title, and some library videos don't have one. Classic Meural Canvases and some older Nimbus frames don't show captions.
To turn it off or change how long it shows: on nimbusframe.net, sign in, click your frame on the home page, and use Art info (Show/Hide) and Show for. Changes save on their own. If the frame is part of a group, change it on the group and it applies to the whole wall. (In the app, Frames > your frame > Settings > Art Info, from app version 3.2.4.)