nRF52832: Why does MOSI stay high after transmitting any byte >7F ?

Using the SDK 15.0.0 SPI example (master) to send a single byte.

Sending 0x00: 

Sending 0x01: 

Sending 0x02: 

Sending 0x04: 

etc ...

Sending 0x7F: 

In all the above, MOSI is low after the byte transmission; BUT ...

Sending 0x80: 

Sending 0xFE: 

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??