Actually QlikView trying to convert to number from text in every possible opportunity, and I reproduced behavior which you described only by explicit conversion to Text() inside ApplyMap() function... You can resolve this issue using explicit conversion to numeric via num() function.
load [Supplier Number],
applymap ('MissingSupplierMap',Num([Supplier Number]),'No Data') as [Supplier]
Hope this helps.
Try to find a definition of MissingSupplierMap.
It should be like
If you don't have this definition you will have 'No data' as result of applymap ('MissingSupplierMap',[Supplier Number],'No Data') as [Supplier]
The second thing is if you have the definition of MissingSupplierMap map, try to load it as a normal table (without MAPPING) and compare the first field with [Supplier Number].
You will see if something wrong.