Mac OS 10.14, Affinity Designer 1.7.2, Mac App Store version.
This is tough to explain, and is only repeatable in that it keeps happening.
I'm working in files that have a large number of groups and sub-groups. Randomly when I am working in a group, other groups below the one I'm working in will disappear from the Layers panel; blank spaces are in their place. I'll ry to get a screenshot of it the next time it happens.
It's usually possible to fix if I resize the Layers panel, and doesn't seem to be causing any bigger problems. It's just disconcerting.