Most of my global smart groups reliably show the number of documents which they contain. Two do not:
In the image above, “Ordner” is custom metadata.
Again, in the image above “Verfall” is custom metadata.
Both these groups only show a document count once they have been opened (ie the contents of the group have been listed in the View pane by selecting the smart group in the sidebar).
It may be a coincidence that both smart groups which do not update contain custom metadata in the conditions, but I think it’s more likely to be causal - all the other smart groups (for which the count works) do not use custom metadata.