There's a setting found at "Tools > Options > General > Advanced" called "Clear recently imported playlist on exit". Have you (Un)checked that?
It's what I thought of at first, but your post seemed to indicate that a second import job within the same MC session was causing the list of imported files from the first import job, or the entire playlist itself, to go AWOL. That's not my experience here, though to be honest, I don't visit that smartlist too often as I prefer to use my own Date Imported view scheme.
As a workaround, you could try adding your own "Recently Imported" smartlist (or view scheme if you like), it all depends how you want to use it:
MC formats date depending upon the system date setting. Have a look at the date in the date imported field. Mine are written like so: dd/mm/yyyy hh:mm
Bear that in mind, then go to "Tools > Options > Library and Folders > User Fields > Add Field"
Name it Current Date
Select "Calculated Data" and paste this string into the template field:
formatdate(now(),dd//MM//yyyy) (ammend the template to match the way MC writes [date imported] for you, keeping the double forward slashes.)
Click OK. Click OK again.
Now, create a new smartlist, call it "Imported Today" and use the the following search rule:
[=isequal([date imported],[current date],8)]=1(click on the import button and paste) This will show all files imported today.
Other rules you might find useful:
[date imported]=<=1d returns all files imported in the last 24 hours.
[date imported]=<=1w returns all files imported in the past week.
[date imported]=<=0.5y returns all files imported in the past 6 months.
If playlist management is your thing, let me know if this interests you:
How does it work?
A playlist group called "My Own Custom Playlists" with one smartlist inside with a simple rule: [media type]=audio
this ensures all your audio files are included in the view scheme.
Press F4 to enter tagging mode. If you click on the "New... " tag box, you'll be prompted for a name, and once you've done that, a new playlist by that name will be created in that playlist group. Sub-Groups work fine too, with a double click on it's pane entry taking you inside the group.
Select some files from the list below and tick the playlists you'd like them to be a part of, or click on a tick to remove a file from a playlist.
It should take no more than 5 minutes to set up, but a bit more than to post it. If you're interested, I'll knock out a step-by-step, just let me know.
-marko.