1.3* micro:bit fails to enter automation mode

With automation enabled, flashing an empty `start_bl.act` file should reboot the micro:bit into MAINTENANCE mode. On 1.5 board variants this works, but on 1.3b board variants, it fails.

**To reproduce on 1.3b**
- Confirm DETAILS.txt file contains “Automation allowed: 0”
- Copy an empty `auto_on.cfg` onto micro:bit while reset button is pressed
- Release reset button while usb drive remounts
- Confirm DETAILS.txt file contains “Automation allowed: 1”
- Copy an empty `start_bl.act` file to `MICROBIT` drive and it should reboot into `MAINTENANCE`
On 1.3b variants it reboots into normal MICROBIT mode.

**Expected result**
All micro:bit variants should facilitate automation using steps listed above.

**Environment**
MacOS 10.14.3
The 1.5 board has Bootloader v0246 and the 1.3b in maintenance has v0234

**Additional context**
- 1.3b exhibits the issue with DAPLink 0241 through 0253. I have also tried this on another 1.3B.
- The manual process of holding reset and adding the `start_bl.act` file does enable MAINTENANCE mode.