RAK4631 doesn't start up after a OTA DFU

I have a image size (.zip distribution) of about 140 KB. I perform a OTA DFU of it using the nRF toolbox app and the operation is successful.
However, the board doesn’t start up or activates the new firmware.

The following is the only way the firmware gets activated and I don’t know why :

1. Upload the firmware image using the nRF toolbox app. DFU is successful.
2. After the DFU is complete, disconnect and reconnect the RAK module from the USB port.
3. It would still not start up / activate the new FW.
4. Now connect to the board’s COM port using Teraterm (or any other serial app). The module would start up with the new firmware. & the serial port start showing logs.
5. Now you can close Teraterm, the firmware keeps running.

PS : For smaller images like about 40kb , the firmware activates successfully after the OTA DFU, without performing any of the steps above.