I looked at this with Trimble Support yesterday and it worked just fine for me - the support tech was also having an Issue but he had his Import settings set to use Point ID and the file he was looking at only had Station, Offset, Elevation so his import was looking for 4 or 5 fields whereas the file only had 3 fields
If your data has an ID it should be the first field (Point ID, Station, Offset, Elevation, Description) and you have to say Use ID in the Import dialog (if you drag and drop it will use the last used settings that may have ID set and if you dont have ID then it will not work or at least work correctly). For these files I use Import command and make sure that I have it set to Import Points or Stored Sections and whether or not I have ID and then Import.
It could be that in 4.12 you have the settings set one way and in 5.0 you have it set differently.
If this doesn't help - call me on 303 635 8556 or send me the file and I will check that it works for me and I will screen grab the settings that I used - we can do a Web Meeting if this is urgent - I am online now
Alan