Here's some screenshots
I was able to duplicate it. Should be relatively easy to fix.
In the mean time, I think it may work if you move the non-cast nicknames to the bottom of the file:
"N400:0","RingoA","",""
"N400:1","RingoB","",""
"N400:2","RingoC","",""
"N400:3","RingoD","",""
"N400:4","RingoE","",""
"N401:0","GeorgeA","",""
"N401:1","GeorgeB","",""
"N401:2","GeorgeC","",""
"N401:3","GeorgeD","",""
"N401:4","GeorgeE","",""
"N400","Paul","",""
"N401","John","",""
Or if you already have NN for the non-cast, it may not import.
I figured out the issue. The default priority for Import Behavior is
Merge (Project Priority). When you import N400 nickname of Paul, N400:0 has an implicit nickname (not explicit, though) of Paul:0, so when it tries to import the NN RingoA for N400:0, it thinks it already has a NN for it (implicit Paul:0 however), hence it does NOT import RingoA. It needs to import EXPLICIT OVER IMPLICIT when Import Behavior is
Merge (Project Priority), which it is not currently doing
So, ANOTHER work-around for now is to tweak the Import Behavior to Merge (CSV File Priority), although that may GIVE YOU NICKNAMES YOU DON'T WANT, since this will also redefine any EXPLICIT nicknames giving CSV File Priority (you may not want that).