First light sensor reading is always duff

[whaleygeek]

Describe the bug
The first light sensor reading always seems to return 255

To Reproduce
Load file measure-fails.hex. Observe that the 'M' dims a little bit after a fraction of a second (this is the light sensing starting on-demand). Press B to read the max reading - it is always 255.

Expected behaviour
Load file measure-works.hex - this has a 100ms delay after the first reading and the first reading is junked. Press B to read the max reading - it is correctly the current maximum reading. (P.S. Don't try this if you are currently flying past the sun in a spacecraft, or sitting on a sun bed.)

Additional context
makecode.microbit.org version: 3.0.17
Microsoft MakeCode version: 6.0.18
microbit runtime version: v2.2.0-rc6

measure-fails.hex.zip
measure-works.hex.zip

Also paging @finneyj and @jaustin in case this is actually a DAL bug.

[microbit-mark]

This was raised on the DAL in lancaster-university/microbit-dal#426

[abchatra]

Dal issue.