For us, Sage 100 2014 Advanced, /C is a miscellaneous comment line item. I suspect even with buffer lines you would still have this issue but it's worth a shot.

If you can add a simple counter value to your line data, instead of setting the L.LineKey, try a temp field set to that column in your data source, and have the temp field set to reset with the line (instead of header).

Also, if your codes *TIME are a fixed list, you may want to set them up as Miscellaneous Items, like /TIME, instead. I think your problem may be an oddity with the on-the-fly item type.

Thanks for all the suggestions!
The workaround of adding "buffer" comment lines between any two similar lines (line that have the same Item ID, Item Type, and Comment) fixed the problem for us. As long as the two similar lines have a comment line between them, the import did not treat them as a single entry and nothing was lost.