for one layer I have an issue with a very few select labels disappearing depending on zoom level. See label "3" on the right in below screenshots. "Scale dependency" is inactive and "Show all labels" is active. This only happens with a few labels in this layer - and always the same. I have another layer that is very similar but based on a different variable and there all labels reliably get rendered.
I found that some labels keep appearing/disappearing just by panning the map around in the same zoom level. I've played around with the various placement options and found that 'placing in whole centroid' vs 'placing in visible centroid' leads to those faulty labels not appearing at all in any zoom or panning state.
Is there a fault with the polygons/centroids?
In another layer using the same map/polygons this problem does not occur.