On a quite new Wikibase Cloud instance, the first thing I did was to set the "formatter url" mapping in Wikibase Cloud dashboard:
{F52907903}
Despite that, the "external id" values of some properties that have a formatter url statement do not become clickable in the interface. Examples: https://eneoli.wikibase.cloud/wiki/Property:P26#P1, https://eneoli.wikibase.cloud/wiki/Item:Q98#P1, https://eneoli.wikibase.cloud/wiki/Item:Q276#P17.
At the same time, this feature does work for a couple of properties, see https://eneoli.wikibase.cloud/wiki/Item:Q276#P22 or https://eneoli.wikibase.cloud/wiki/Item:Q276#P43.
In all cases, the https://eneoli.wikibase.cloud/wiki/Property:P2 values have been set to the properties before populating the Wikibase instance with data (=it is not newer than the statements that should become clickable).