This indicates that the allocated data update for the upcoming Solah has been depleted.
The iman app will schedule timeline updates each time it runs. If the app hasn't been used for a while, the allotted timeline will be depleted, and future timelines won't be displayed, rendering the countdown invalid or counting up.
Another method to prevent this, even without opening the app, is by granting location permission to "Always". This enables the app to update timelines in the background without user intervention.
However, this depends on user movement. If the device remains stationary at the same location for days, there won't be any location updates. Typically, though, daily commuting from home to work provides enough movement for the app to refresh itself periodically.