First, visually complete relies on the Mutation Observer API, which is only supported in modern browsers and IE 11.
If it does not apply to the case, most likely the action was closed preemptively (e.g. due to a redirect), or there simply were no visible elements on the page.
To make actions easier to compare, Dynatrace only sends visually complete if we can guarantee that the calculated value is meaningful. If the page was redirected, Dynatrace has to abort calculation, thus the value is meaningless.
This also applies to XHR actions: if an XHR action does not cause any mutations on the page that included visible images, it will not have a visually complete value.
