You see the field in the import dialog box, and it's set to import (you see an arrow between it and the target field), and you can use the left/right arrow to see the data, and yet not only does it not import, but all the field mapping past this field is shifted compared to what was set in the dialog?
All Source Fields arrows are correct and records can be single stepped through using the Record chevron in the lower left of the dialog box.
All Source and Target fields are set correctly.
I was hoping someone else would jump in cause I'm not at all clear whats up. I hate jumping to corruption but maybe try running restore on the file to see if it spits out some bad news.
What type of import is this (add new, import matching, etc.)?
Does it say at the end of import that all records were imported (it tells you the number of records imported)?
Is this part of a script which could have some funny business going on after?
Maybe to further isolate: try importing a really simple text file with "Hello" in it, and mark it as a .csv, then import that into your one problem field... just to see if you can get anything to go in that field.
I also think there are almost certainly better ways to fix your serial numbers. If you tell more about that problem we can surely give some advice there, but at this point I'm a little worried about your file!
Try re-saving the file as xlsx
I had issue with excel that were solved by just a re-save as
Try a manual import to see if it works.
Try a brand new import records script step.
Try a new script.
Try recovering your file to see if you have the same issue in a recovered copy.