completed work units shown until the next is completed

This isn't a bug.

The BOINC upload sequence is in two phases:
1. The actual finished result file is uploaded to the project, to be saved on a hard drive on the upload server.
2. The result is reported to the project, meaning it is written into the database that you finished this task.

Uploading the file takes neigh on no overhead, while reporting it to the database does take a lot of overhead. Yet because reporting more than one task at the same time to the same project adds no overhead, BOINC waits up to 24 hours with reporting the task as it's possible you have more than one task of that project to report at the same time.

For BOINC 5.8 and above: Completed work is reported at the first of:

1) 24 hours before deadline
2) COnnect Every X before deadline.
3) Immediately if it is later than either 1 or 2 upon completion of the task.
4) 24 hours after completion.
5) On a trickle up message (CPDN only, I believe).
6) On a trickle down request.
7) On a server scheduled connection. Used, but I am not certain by which project.
8) On a request for new work.
9) When the user pushes the update button.
10) When using the <report_results_immediately> option in cc_config.xml it will happen 1 minute after uploading. (requires BOINC 5.10.45 or later)

Now please, in the future ask things like these on the project's forums or the BOINC Dev forums. You've stumbled onto the BOINC Bug database, which is for reporting bugs, not for asking quite standard operation questions that can easily be found through Google.