Why does Accept statement change least-significant decimal place to 0?

When I build the following program with GnuCOBOL 3.2 on macOS and enter 10.5, why does it print 10.0?

       program-id. foo.
       data division.
       working-storage section.
       77  x   pic 99v9.
       procedure division.
           accept x.
           display x.
           stop run.

Similarly, if I change the Picture clause to PIC 9(3)V99 and enter 458.12, it prints 458.10.

The picture character V is an implied decimal point. Therefore your first example does an implicit MOVE of 10. The invalid . is replaced. Similar for the longer variant.

What you can do is to ACCEPT a bigger PIC X field and then MOVE FUNCTION NUMVAL (INP-FLD) TO x which then uses the implied decimal to position it correctly. This also has the benefit that you can use a sign along with the S picture character, which in your example of a USAGE DISPLAY variable will do an "overpunch" of a character to store negative values.