DFU has been failing recently. Yes, it was working fine until a while ago.
Specifically, the screen that was displayed while waiting for the device to reboot after DFU disappears immediately. That screen is the one on which Validating is displayed.
Originally, DFU would be completed by waiting for a reboot on this screen and reconnecting again. Otherwise, a rollback will occur and the DFU will fail. The nRF Connect App times out because the screen disappears without waiting for the device to reboot, resulting in a firmware rollback.
For reference, when I use AQUOS Phone, DFU succeeds. So I feel that it is not an SDK issue but a smartphone issue. But is that possible?