In many organizations that utilize SharePoint, the Human Resources (HR) administration is an (one of the) origin from which data is propagated into the User Profiles. The presence of HR data in SharePoint User Profiles then allows to retrieve that data for usage in SharePoint API based applications, e.g. JavaScript (vanilla upto SPFx), InfoPath, SharePoint Workflows, Microsoft Flow, ... An issue here is that in case of custom User Profile properties, the User Profile REST service does not support to return their value: Query SharePoint user profile by custom property. Approach to deal with that lack is to retrieve/query the custom User Profile properties via User Profile Search: Query and filter user profiles based on the value of customer property using REST.
Consequence of retrieving via Search is that the returned data may not be the actual value, but is as fresh as the latest crawling of User Profiles as search source. In case of an on-premise deployment, an organization has self-control over the crawling rate; default setting is a full crawl per 15 minutes. Implication is that typically the data cached in the SharePoint Search Index is not older / outdated than maximum of 15 minutes. This is for the most business usages acceptable, as HR data is not updated continuously. However, in case of SharePoint Online usage, matters are different. Microsoft makes no concrete statement / promise on the period it takes to have changed User Profile values become visible in SharePoint Search. On experimentation, I observed that for one User Profile instance a change in custom property was visible + retrievable via Managed Property within 30 minutes, while for another User Profile instance a change in the same custom property was not picked up yet after a period of 3 days! This unpredictability makes the utilization of User Profile Search to retrieve User Profile property values less reliable.
I've consulted Microsoft Support on this topic. In a nutshell the response is that my observations is the current way of working, and that Microsoft cannot give an hard statement on the maximum duration in which User Profile property changes will be visible in SharePoint Search. And current in SharePoint Online context, one does not have the option to explicit request for reindex of User Profiles via full crawl. Microsoft themselves refers to a SharePoint UserVoice request to give your vote: Reindex User Profiles option.
Mikael Svenson posted a workaround that allows you to get "kinda explicit" control: How to trigger re-indexing of updated user profiles in SharePoint Online. Although for multiple reasons certainly not the preferred way, until Microsoft delivers on the UserVoice raised request, this approach is likely the best available to get changes in User Profile instances at least within a delineated elapse time visible through SharePoint search.
