Thank you for your post.
I have not seen this manifestation previously, and I realize that doesn't help you.
Do you see this issue immediately? Or, does this occur after an extended period of time?
With these records that are "missed", do you notice if the records are recently entered or if the records are old, or if the records are random?
If you create a new layout with a new portal displaying the same records, does the issue still occur?
In essence, I'm trying to determine if something changed in the file either with the data or the layout.
Run a Recover (File -> Recover) on the file and see if any errors are reported.