This happens when a frame renders too long and AE just can't make heads and tails of the render time. This behavior has existed since the earliest days of AE. Typically it's some plug-in that doesn't correctly report its processing time. Anyway, given that you are clearly "abusing" AE to render long-form content which it was never meant for it is to be expected that soem values will be reported wrongly because some values exceed the internal limits and won't update after a while. Just let it render. At some point it will just magically finish without any prior notice.