What’s the default baudrate?
I’ve had a look at the RP2 port of the machine library (are you using an RP2040?)
The SPI section looks like it defaults to 10MHz which might be a bit intense for a breadboarded circuit. If the speed is too high the signal integrity will be compromised (the transitions become mushy).
Try something lower eg.
spi = SPI(0, 100_000) # Default assignment for SPI0, 100kHz
It might still be helpful to be explicit with your pins, just in case. Leave nothing to chance 
