Planned specimens, defined in a CP Event, are hidden after a certain number of days if not collected. This is done to reduce the clutter in the user interface.
By default, this is 60 days and can be configured via Settings. You can increase the number of days to a large number if you do not want to hide the pending specimens.
For setting up '0' days or an empty value, OpenSpecimen hides pending specimen immediately after collection of the visit.
If the user wants to show pending specimens all the time, the user needs to set some more significant value in "Pending Specimens Display Interval" like 36525 days (i.e., hide after 100 years).
From the Visits page, you can hide/unhide such pending specimens using the More->Show Pending option.