File modification date is always rounded off to nearest hour in past, update is performed only if previous timestamp is different from current value after rounding off.
MODIFICATION_DATE : Date of the last open/close for writing, rounded back to the nearest hour
This column actually it shows only an access time. V$ASM_FILE metadata table only gets updated at file open and close time.
Check this MoS note:
ASMCMD ls -lt shows wrong date/time for control and data files (Doc ID 1603991.1)