Using the SDK 15.0.0 SPI example (master) to send a single byte.
In all the above, MOSI is low after the byte transmission; BUT ...
So, when the MSB is set, MOSI goes high after the after the byte transmission - even when the last bit transmitted was low!
What's going on here??
Seems like it's getting ready to send another byte (same one), loading the MSB.
Have you tried transmitting data on SPI using registers directly instead of the driver, to see if that makes any difference?
The suggestion from agus.uy sounds reasonable.
No - this is just taking the Nordic example as-is.
In the OP, I'd changed the buffer size to 1 just to make it easier to see.
I get exactly the same effect if I make no code modifications at all, and just edit the last byte of the buffer in the debugger