Why do my Virtual Turns (SPA Pages, Custom Timers, and Mutation Observer Calls) have no Performance Detail metrics?

Custom Timers/SPA Pages are not considered full page turns. These implementations of Virtual Turns are generally defined by start and end timers. (SPA Timers being implemented on the BTT side - Custom Timers implemented on the customer side) Mutation Observer looks for changes in the DOM to trigger page naming rules.
Since all of these Virtual Turns do not occur on full page loads, they do not come with any Core Web Vitals metrics. Often times you will just see Onload as the displayed metric. This Onload value is not a true "Onload" event, but is more of a duration of how long it takes to get from the start to the end timer. Custom Timers are recommended, as these are often more easily able to monitor specific calls.